I downloaded GZDoom and whenever I try to run it I get this fatal error:
After adding doom2.wad to the config directory it booted but now there's this strange green bar covering my screen, and I'm getting under 10 fps which is strange for a game from the 90s. This computer is from 2008 so i don't understand what could be the cause of these errors
gzdoom
is a game engine and does not include the game data required to actually play a game.You must place the game data in the directory specified in the error message (or symlink to it, which is what I do). Alternately, also as mentioned in the error message, you can edit the config file to point to a different directory containing game data.
If you don't own a copy of the game(s) supported by gzdoom, you can download completely free game data that works with gzdoom, for example freedoom