I guess it depends on exactly which packages you installed to begin with. gnome-desktop doesn't bring in an X server, so you must have chosen some other package(s) also in order to get an X server at all. Installing the X Window System group will install the vmmouse driver, but installing only the less-expansive xorg-x11-server-Xorg package and its direct dependencies will not... you'd then need to install the xorg-x11-drv-vmmouse package directly or through some other metapackage (e.g. xorg-x11-drivers) or dependent package.
Your guest's /var/log/yum.log should tell you what was installed, as long as it wasn't too long ago that the logs have been rotated away.
Cheers,
--
Darius