Gianna is alright, but to me it reads read as unfinished - less of a nickname than a name in its own right.
I have no problem with
Natalie by itself though it has been overused, I think. It now seems to be slipping past its peak, which is good except for the fact that it could come to be seen as a dated trend name. I think it's classic enough to avoid that fate.
As middle names, Brie and
Brielle are both non-starters for me. Brie is a cheese. It's fine as a nickname, but not a given name. (I feel only slightly less harshly about
Bree.) I have never understood
Brielle as a name, frankly. I think it's unattractive and doesn't really do much to shorten its parent name. So combos here are definitely not my kind of names, because I detest both middle names.
Andrew
put a smile on your face - make the world a better place (: