Definitely won't need to reinstall. Just use whatever package manager comes with openSUSE, and install Gnome. It will automatically install all dependent packages that are required too. Is it called YaST in openSUSE?
Then, at the log-in page you have an option where you can select which desktop environment you want to use. Just select Gnome instead of the default KDE. I think it's called Session, as opposed to desktop environment... bottom left of the log-in page, iirc.