DRD Team

Concerning GZDoom, is it legal to release a completely new game using gzdoom and a homebrew iwad? what legal stuff do i have to include (this is not for profit BTW) if i have made modifications to the GZDoom source or other contents like GZDoom.pk3?

You can do that but you are required to release the source along with your project.

However, first I'd suggest to show me the changes you made. Maybe they can be included into the regular version (if they are useful and not too specific) so that you don't need a custom EXE (which nearly always will kill a project sooner or later if it can't keep up anymore.)

i havent made any modifications yet. i planned to, but you're right, it would be hard to keep up with the official version. however, i have experimented with modifying gzdoom.pk3, if i release a new version of gzdoom.pk3, what legal stuff applies to it?

I strongly recommend not to do that. Put your stuff in your own WAD. There's absolutely no need to replace gzdoom.pk3. Everything in it can be replaced with external files.