If you aren't already using it, there is a good program called
SWareT (meaning SlackWARE Tool) that can be used to manage packages with Slackware.
I'm sure you can use that to reinstall gnome without having to reinstall the entire system.
Make sure you edit the
/etc/swaret.conf file per the documentation to point to a Slackware version and location that is correct for your install ... otherwise you might get upgraded / downgraded to a new version.
In fact, after you get swaret setup for your install, I think the commands to get you working will be:
swaret --update
then
swaret --dep gnome
or
swaret --reinstall gnome
or
swaret --install gnome
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If you want all the latest patches/updates do:
swaret --update
then
swaret --upgrade