How is it credited on-screen ?

There are no credits within the show. Gunbuster's theme, characters, setting, voice actors, plot points and message were prototypal versions of Neon Gensis Evangelion's, which was written and directed by Anno. It's very evident he also made Gunbuster as he's also known for creating the vast majority of Neon Genesis Evangelion. He forgot/didn't bother to put credits in Gunbuster because it was his experimental debut working with a small animation team for a short OVA anime.

If there are no on-screen credits, as per our rules, the field should remain empty, even if there are informations elsewhere.

If there needs to be on-screen credits in order to properly credit it, then none of the details about the show should be detailed either. It has no credits. All the current credits are based on sources outside of the show. You either add Anno as the creator or you lock the entire show's details off and mark them as empty as well.

@Kavihira said:

It has no credits. All the current credits are based on sources outside of the show.

Both the Gainax website and the physical packaging feature a credits list. Wouldn't call that "outside", it is as official as it gets and totally to be considered in these situations when there's no on-screen credits.

Neither have creator credits either. So the field remains locked.