Aardappel has .map sources of his
maps on his site, and in IRC many years ago while 'adopting' them for OA early on (0.0.5) he told me they're GPL, in a PM. The zip file's text says 'educational use' however. This can be confusing for some as it would bring up a lot of "[citation needed] you violator" accusations. He should like reupload the archive with a COPYING in there.
Now i'm returning to those maps and i've souped them up with Freedoom textures and his supposedly public domain 'i don't care even if i don't get credit even' texture pack, and they're looking good