I honestly don't know. Can only speak for myself and I'm a Monster but I've never been a fan of Christina. I never liked her make-up or the way she dressed. And although she has a great voice I wasn't the biggest fan of her songs because I don't like that thing she does with her voice (don't know how to say it in English,but you know what I mean,the way draws out the notes to eternity instead of just keeping it short)
Well, once they collabed with Do What U Want -duet, both fanbases had something to celebrate together.
Moreover, they do both have talent, so there's some common ground. Christina frustrates her fanbase by being MIA for eternity and then randomly dropping some Birthday Party pics or smth like that. Gaga on the other hand frustrates some of her fans with messy era(s). Both have potential but don't always live up to it.
I don't know if Monsters and Fighters are friends on ATRL, I guess it depends on the individual people.