ZDoom

It seems pretty common for people to delete the "zd_extra.pk3" or "game_support.pk3" file from their GZDoom distribution thinking they don't need those to run Doom or whatever.

Unfortunately - you do. If you have deleted these files, try putting them back. Your GZDoom might start working then.

The purpose for this file being separate is so that indie game devs can remove it in their distributions in compliance with copyright and the commercial exploitation thereof. It separates a lot of the content that is meant for the games that GZDoom is meant to run in its default distribution state which includes this file. So naturally, removing it means GZDoom can't run those games anymore.

Adding to that, the reason why IWADINFO has been moved there is to keep the commercial stock IWADs away from such indie games. A common problem has been that every indie game using GZDoom as its engine was picking up any installation from GOG and Steam and unwantedly popped up the IWAD selector. And since the game_support file is needed anyway to play the regular games it came as a natural solution to put the information about these IWADs there as well.