Here are the instructions to use GFS repository through slackpkg+

1. Download and install slackpkg+ package from http://slakfinder.org/slackpkg+/
2. Add gfs on REPOPLUS line so that it will become like this
   REPOPLUS=( slackpkgplus alienbob restricted slacky gfs )
   You may remove other repositories (except slackpkgplus) if you don't use it 
3. Add gfs repository information and match it with your Slackware and 
   GFS version, and your architecture
   Example:

   For Slackware 15.0 (64 bit) and GNOME 41.9:
   MIRRORPLUS['gfs']=https://slackware.uk/gfs/15.0/41.9/x86_64/
   MIRRORPLUS['gfs']=https://reddoglinux.ddns.net/linux/gnome/41.x/x86_64/

   For Slackware Current (64 bit) and GNOME 42.10:
   MIRRORPLUS['gfs']=https://slackware.uk/gfs/current/42.9/x86_64/
   MIRRORPLUS['gfs']=https://reddoglinux.ddns.net/linux/gnome/42.x/x86_64/

   For Slackware Current (64 bit) and GNOME 43.4:
   MIRRORPLUS['gfs']=https://slackware.uk/gfs/current/43.4/x86_64/
   MIRRORPLUS['gfs']=https://reddoglinux.ddns.net/linux/gnome/43.x/x86_64/

4. Run slackpkg update gpg && slackpkg update && slackpkg install gfs && slackpkg upgrade-all
5. You can start managing your GFS packages through slackpkg+