Hm no idea what I'm doing wrong here...
Went into boot/config/plugins/community.applications/private/
created a folder:
DiamondPrecisionComputing/
went into it and wget your Gluetun template (wget https://github.com/DiamondPrecisionComputing/unraid-templates/blob/master/templates/GluetunVPN.xml)
Even rebooted just to be sure.
Now I go into the apps tab and I still don't see a "private" section as mentioned in your link above. going to that folder via terminal I see the .xml file right there..