gigCal_gigs_css_classes
To properly use this filter, you have to register your function a little differently than normal. Passed along with the array will be an object for the Gig. This will allow you to access all of the information that the plugin stores about gigs. When you register your hook, it should look like this:
add_filter('gigCal_gigs_css_classes', 'mytest', 10, 2);