Thing with GUMI is she's owned by Internet Co, so whilst they got her for Mirai DX even there she was kind of limited (not available as a roommate etc). They'd need to negotiate rights and all that for her to show up in DIVA.
I did read some stuff on my feed (IIRC from one of Internet's blokes) suggesting if there was enough demand they'd consider it, but for the time being I doubt it, even if they're adding plenty of Mirai songs nowadays. That said, even when DIVA technically does feature songs with GUMI they go for a Crypton alternative (1, 2 Fanclub shows up in DIVA X, but it's Giga-P's Miku and Len version).