Fanservice (サービスカット) is content in an anime or other work that is meant to please the fans. This is usually achieved using Boobs or Butts.
Since Gunbuster and Diebuster are OVAs, they can get away with showing boobs uncensored, thereby increasing their power.
The concept of Topless is surprisingly unrelated to fanservice, at least when Nono isn't getting the two confused.