Same issue here.
Using Gantry framework, and Cerulean Rockettheme template with Joomla 3.0.4 & K2 2.6.2

As soon as K2 was uninstalled, I had access to the template again.

I have the same problem. Joomla 3.0.2. K2 2.6.2, Rocket Theme Cerulean template 1.0, Gantry 4.1.4

If I unpublish the System - K2 plugin I can then access the template from the backend.

When you look at k2.php (the system plugin) you can see that the K2 devs are using

JRequest::getCmd()

which is deprecated.

They should use

JFactory::getApplication()->input->get() instead.

docs.joomla.org/Potential_backward_compatibility_issues_in_Joomla_3.0_and_Joomla_Platform_12.1

Workaround: Just select template name with checkbox and click on Edit button. That will do the same as if you clicked on template name ...

Perhaps "one day" the K2 guys will change that (and stop blaming Gantry) ...

I guess you could also just "search & replace"
JRequest::getCmd with Factory::getApplication()->input->get
in the k2.php ... but I haven't tested that.