Gail has presented Etta and Diana as though they were super-tight bosom buddies who've been aw shucks besties 4 life! for years. This take on their relationship has precisely zero foundation, and no emotional resonance, because the last twenty years of canon has demonstrated quite clearly that there are easily hundreds of people with whom Diana is closer than she has ever been or probably could ever be with Etta.
Similarly, Gail has presented Giganta and Diana, who have pretty much no relationship whatsoever and have fought like, four times, as long-term enemies on a first name basis with significant emotional investment in each other (negative though that emotion may normally be). This equally foundationless emotional beat rings equally hollow.
That said, with Giganta at least, Gail's hardly the first - writers have been talking her up as significant to Diana way out of proportion to her actual canon since Jimenez first gave her that body. So in neither sense should it be surprising that they're on a first-name basis here; that's both typical of how Giganta gets treated, and typical of Gail's run in general.