I recently installed a rockettheme template and gantry onto my existing Joomla website which is using T3 and JoomlaArt template.
ucfgsa.org
Oddly enough even when I made sure the JA template is default and gantry plugins are disabled, its still generating this error when you visit the website. I don't want to uninstall gantry and Callisto as I'm preparing them for the next version of the site but at the same time this clearly makes it difficult for people to access my current one.
It's most like that you have the menu item (which is default) set as a Gantry 5 menu item type. Check that.
If you need more help, Please would you post your URL, superuser id and pswd in the secure tab of your post and i'll have a look for you.
Regards, Mark.
Please search forums before posting. Please make sure your post includes the version of the CMS you are using and a link to the problem. Annotations on screenshots can also be helpful to explain problems/goals. Please use the "secure" tab for confidential information.