First do a "pkg_delete gnome2" or gnome2-lite or whatever you installed. Then you can perform the usual "make install clean". The port manager won't let you double-install, btw. You will get an error message that gnome2 is already installed.
Which version of gnome do you have installed? You may want to look at
www.freebsd.org/gnome and look at the faq to make sure you have the various daemons and things installed properly.