If you can boot to the command line, then you should be able to recover.
Check your /etc/slackpkg/mirrors file to ensure that you are pointing to an appropriate mirror for the version and architecture (32bit or 64bit) that you want. The upgrade has likely installed a new mirrors file that needs to edited.
Then try 'slackpkg update gpg' again. If it still fails, I would manually download and reinstall the slackpkg package.
For further details, see
http://docs.slackware.com/slackware:slackpkg
Note the steps shown for a full system upgrade.
PS - This thread would be better in the Slackware forum.