Wow, I can't even EXPLAIN how much I love HSM. Anyway, I think that Gabriella is sort of a sweet smart girl, very much like Belle, so perhaps you could sing "Home." It's pretty and suttle. I actually think that Gabriella is a Mezzo, so maybe you could sing a song with a little bit lower range (depending on your range, of course). Oh.
I actually just read the end of your post with the whole "pop song" bit it in. If you sing a poppy song, people might think that you want Sharpay, seeing as she's so much more outspoken and drama-queen-y.
Well, whatever you choose, hope you have a great audition! <3