I think Brianna lends itself better to Bria than Gabriella. I think it's fine as a full name though. I had a student named Bria and never thought it was overly nicknamey/juvenile.
My niece is named Gabriella and we call her Bri. So I could see it with nn BRIA . I am partial to the name Gabriella though. I love it and think it would be very pretty!
Re: love a nn, need a full name!
Another one could be Sabrina