Using Joomla 1.5.24 Gantry-based templates with K2 2.5 causes an important problem with the front-end editor function. The 'Edit' link will break, preventing activation of the front-end editor if Build Titles Spans is enabled in the template parameters.
So, to use the K2 editor with Gantry templates, you must disable 'Build Titles Spans' in the template parameters. This is current up to version 3.1.17 of Gantry.
Hope this saves effected people the hours it took me to figure it out
I had the same issue. K2 front end editing is turned OFF by default. I found this article on another site with instructions how to turn it on. Once turned on, works very well. Hope it helps you.