Even if it's by a fanartist. Gainax sometimes likes to reward artists doing fanart of their shows by granting them a gainaxtop pass. Your work hangs on their greeting-page for a week then.
Well that's pretty cool of them. However, it doesn't explain why it's copyrighted by Gainax. Also, surely if it's copyrighted by them, that makes it official even if it was originally by someone who isn't a Gainax employee? The NGE figurines aren't hand-carved by Yoshiyuki Sakamoto and yet they're still official since they're licensed by Gainax.