It's Gaga.
She loves her and I found it weird that in all the interviews, she didn't mention ha, when she was asked about music and so on. (I am sure she would have mentioned her tbh, she mentioned Ed, Rihanna, Beyonce, Madonna and many others).
Plus this:
http://atrl.net/forums/showthread.php?t=833200
It's not Bruno

Adele talked about collabing in the studio with Bruno several times.
She never mentioned anything about Gaga, even tho she sent her flowers, attended her shows (the one private event where Gaga sung Poker Face acoustic with a big wig and Itunes Festival) and then went to her studio in Malibu, this January + played her Hello
It's obviously her. And I
hope have a feeling it might be on LG5 hence why she don't want to spoil anything.