You'll only have the option to boot into gnome if you've got it installed.
Not sure how you'd check (other than logging out/ending session and then restarting).
Surely you should have a list of rpm's available for install? It's been 2 and a half year's since I last tried SuSE so I don't recall what is shown by YAST. Mandrakes software manager show's a list of stuff that's available for install and a list of stuff that can be removed (because, suprise suprise, it's already installed). You could look to see if YAST offers you a similar type of listing.