When I installed SUSE I chose Plasma as the desktop environment. I remember installing the files for Gnome as well, assuming that I could switch 'twixt the two after installation (I remember doing this easily in past versions/distros of Linux); however, I can’t find anywhere how to switch. Is this possible, or must I reinstall the OS?
For sure you don’t have to reinstall the OS. If you have all the Gnome packages installed switching is definitely possible. In 13.2 there is such option on the login screen (before you enter your username/password). You might need to change your login look to see those options.