So I play on the GOG version of Ultimate Doom, and I can't for the life of me figure out how to boot up this custom wad i downloaded. I'd appreciate any and all help you can give, so long as it doesn't involve me downloading a source port (I like the GOG version just fine :) )

Open the dosbox configuration file and append "-file path_to_wad_file" to the line starting doom.exe.

Ping me if you need more verbose instructions.

All of the older ones do still work and you can still find them somewhere, and the post above me has the method for that. I believe AliensTC and Batman Doom are prime examples here. Oblige should also make WADs for the original as well, though you need to make sure the engine is selected.

Hate to break it to you though, but most WAD files these days run off of a source port such as ZDoom because it removes limits imposed on modders (and because they've been around for so long). Using that it's about as easy as dragging the WAD on top of ZDoom's EXE. Chocolate Doom will help with this if it's compatible because it tries to be as close as possible to the original (I remember a game made based on that one recently). Not trying to push you into one but just a friendly warning: if it specifies what port it's made for, the dev means it.