Writing for Gathox
I use a few different tools for writing. I've met many die-hards who proclaim that one word processing program is all you need, and futzing around with anything else is just a distraction. Honestly, if you believe that, then that's great! You've found what works for you! Generally, I like to switch things up depending on what phase I'm working through in terms of process.
The Master Notebook
That's right, a ton of my initial bits of writing happen in my master game book, which is basically just a classic composition notebook filled with square-gridded paper. Dungeon designs feed into writing which feeds into tables and so forth, and the graph paper makes this process just seamless. I tape little flaps onto sections of the book to separate my ideas and maps, and also happens to be an organic way to organize stuff after it's created. I really believe no GM should run a game without it.
The Distraction-Free Text Editor
You know how the Game of Thrones guy is famous for writing his books in old school DOS? He does that to keep distractions down to a minimum and focus on content generation. DOS is whatever, I have fond memories of it from my youth (writing Basic-A programs to generate characters and monsters, etc.), but I don't touch Microsoft stuff unless I have no other options. I use Linux exclusively, but these options are cross-platform to the best of my knowledge.
Focuswriter is a distraction-free text editor which hides it's interface unless you mouse over it, drops in a pretty background across your entire screen, and lets you just write. You can do word count, spellcheck, export to a decent selection of formats, and customize how it looks. A lot of folks swear by it, and I've clocked plenty of productive hours in it.
PyRoom is another distraction-free text editor that I've spent hours writing in. It looks like a Matrix terminal, can shoot typewriter sounds at you if you're into that sort of thing, and basically does everything Focuswriter does - namely, block out the nonsense and let you just simply write.
My favorite distraction-free text editor, though, is the blindingly fast, tiny overhead, amazing terminal word processor called Wordgrinder. This thing has all the features you'd expect and it runs straight in a terminal, which means you could use it on a computer from the 80's and it would still be Johnny On The Spot. There's just something sexy about doing creative work in the terminal, and honestly I don't see myself ever giving up on this tool. You can use it on OSX's terminal, and in Windows you can use it in Putty or whatever sort of terminal with bash implementation they have in Win 10 (I heard they have something new there, but that's none of my concern).
The Big Office Program for Edits and Formatting
I use LibreOffice, which is a fork of OpenOffice. Unless you're dropping cash to Microsoft on a regular basis to use Microsoft Word, you've likely used one of these two programs. LibreOffice is a bit newer, with a larger group of folks working on the codebase, and it does every single thing you need an office suite to do. Edits and suggestions carry over nicely from Google Docs and from MS Word edits, and document versioning is easy to control. It can handle hordes of different document types and extensions. It isn't exactly a small program though, and if you needed a lighter substitute for an aging machine or you just don't want to waste the RAM overhead because of heavy multitasking, I would suggest AbiWord. AbiWord also looks pretty, so there's that.
The Big Layout Program
Hey, if you're sold and set on the Adobe office suite, I'm not here to dissuade you. But, having worked with Pagemaker, In-Design, Corel Ventura, and Quark Express over the years on various platforms, I have to say that Scribus has all the tools I personally need to get layout done. Now, I really don't like doing layout and when possible I will offer someone else money to do it, but sometimes you gotta layout a bunch of pages on your own and get the job done. Scribus handles all the things you would expect it to and even handles postscript stuff pretty solidly (with ghostscript).Also, if you're publishing through any of the OBS sites, they've got tutorials for doing it with Scribus.
Art-making for Gathox
There's nothing that beats a pencil and some blank paper - if you're not starting there with your art career, you're selling yourself short. And quite frankly, there are a number of illustrations in Gathox Vertical Slum that are nothing more than fully-rendered pencil drawings that I scanned in and cleaned up. I suggest using some 0.9mm mechanical pencils with #2 lead and some typing paper. Computer paper? Whatever. For finished pieces I use cold press watercolor paper or bristol board, but basic reams of white paper will get the job done. Use a soft white eraser or a gummy eraser - the pink ones are paper-tearing garbage and should be treated as such.
I'll ink pencil work, usually with a crow quill and/or a thin rigger brush for more expressive lines and fills. Once in a while I'll switch to a set of Microns if I'm going deep in on hatching or contour lines, or if I need to draw a lot of architectural stuff - the Microns excel at not bleeding everything to shit when drawing hard lines with a ruler.
I often mix up a wash with my permanent ink and some water if I want some tonal fills or more natural looking shadows. Many people are better at this than I am, but I keep at it and am steadily making progress.
For color painting, I'll use either gouache (for quick color studies or brief finished pieces) or acrylic. With acrylics, I often use open acrylics to keep my canvas able to do lots of wet-on-wet technique. I personally do not mess with oils; if I need that oil painted look, I'll either get my acrylics close to that look through a combination of wet-on-wet and glazes, or I'll do it digitally. I know that someone reading this is screaming inside, but I just don't need the literal and figurative headache of oils right now. Maybe someday . . .
Hands down I choose the GIMP as my raster graphics program to clean up my scans, beef up my black line work, and stitch together a large piece from multiple smaller scans. Unless you're already dropping hundreds or thousands on Adobe products, there's no reason to live without this tool. A lot of people make great art in it as well, but for digital drawing I have other, more specific tools.
Digital Drawing and Painting
Krita. Krita Krita Krita . . . Krita. This thing is a powerhouse! Krita has 13 brush engines which can be combined into multibrushes and do multi-layer multibrush painting. It can emulate natural media or do weird, digital-specific kinds of rendering. It handles all the layer effects you'd expect from PS or anything else, it can handle animation, some vector stuff, and has tons of perspective and layout tools. Also, it's got a subsystem for setting up comic books, which is pretty cool. I cannot suggest this enough, it has literally made me tens of thousands of dollars over the last 5 + years. Also, it can share files with GIMP and Mypaint through a special filetype called .ora (open raster archive).
If you want an infinite canvas, a nice selection of easily-accessible brushes, or you just really want to do some digital artwork but don't have a beefy computer to handle Krita, I would suggest Mypaint. You can get really nice, layered illustrations done without a lot of RAM overhead. You may need to trim your pieces in another editor like GIMP, but Mypaint really is a useful tool. I have it installed on my 900mhz, 1 GB RAM, 8.9" wide Asus eeepc netbook, and manage to get sketches done on that piece of super-underpowered hardware with Mypaint and a little $80 Wacom tablet.
Inkscape is my go-to for vector editing, whether it's putting together a poster for a con event, tracing a bitmap to vectorize a drawing for a logo, or building dungeon maps that look nice. If you don't know what vector art is, it's basically art that maintains resolution at any size because it is rendered with points and calculus instead of colored pixels. Not great for everything, but indispensable for design problems.
In putting together a campaign setting and actually running countless hours of Gathox, I've come to rely pretty heavily on some traditional tools. Others are just things that help with GMing in general.
The Master Notebook
I had to mention it twice - if I lost my master notebook, the one with the graph paper and dungeon notation taped to it and my homemade tabs, I would be lost. Most sessions I can just flip this thing open and run a dungeon or ten. Given that GMs pretty quickly establish their own shorthand and notational conceits, information becomes super densely packed in the master notebook. Something that looks like a simple dungeon level and some brief numbered lines might actually constitute dozens of hours of gaming and just as many pages when fully-written. If you don't have a master notebook (preferably filled with graph paper), you might just try it.
The Big Burly Binder
This is sort of a slush pile of things, from extra character sheets, hireling cards, maps, and player aids to an archive of dead characters, rules revisions, design notes, sketches, and player feedback. Every so often I go through and flush it out, moving things out of the binder and into long term files, but I keep adding new bits. And really, I can't think of a GM who couldn't or doesn't find use in a Big Burly Binder, or something very much like it.
The Session Tracker
I keep many copies of this sheet handy, because this is really where the rubber hits the road. It gives me a handy Death and Dismemberment table, allows me to track all of the ailments of the PCs, do marching order, keep track of time, and make notes on player needs and campaign changes. I've come to rely on it so much that the last time I had to run game without it, I manually rewrote the entire thing on paper beforehand. I hope you find it useful!
The Whiteboard/ Really Big Paper
Having a giant pad of paper for the entire table to see is amazing in helping to quickly visualize battles, spatial relationships, allow for player planning, and to track marching order or anything else. Also, sometimes you have to draw large genitalia at game to keep the . . . juices flowing. Also, you can use it to do the math to figure out who's buying beer this time.