Freedoom haves a lot of errors

It seems like GZDB is trying to find some Doom 2 sprites but can't find them in the Freedoom iwad, shouldn't matter, you should still be able to map just fine.

Make sure to use freedoom2.wad as your IWAD.

It's okay now, I'm using both wads, if I just use the second one there would be some monsters and weapons that wouldn't load (the super shotgun for example)

Look, Freedoom: Phase 1 (freedoom1.wad) is a replacement for Doom 1, Freedoom: Phase 2 (freedoom2.wad) is a replacement for Doom 2. So if you want to create maps that are compatible with Doom 2 you should use freedoom2.wad as your IWAD, and only that. And for compatibility with Doom 1 use a Doom 1 config in GZDB, and freedoom1.wad. Loading both freedoom1.wad and freedoom2.wad as your resources will allow you to use textures from both, and you might end up with maps that have missing textures if run with only the IWAD you designed the map for.