To set up free events, first create your event, and schedule your event by manually creating an event Instance in the appropriate venue.
Then set the Performance Type to Free as follows:
By default, buy ticket buttons will be labeled "Free" or "RSVP" depending on what you select.
RSVP events will typically have a custom Ticketing URL which points to an online form where the patron can RSVP or sign up.
In Groundplan Theme, you can further customize the button text with this field:
If the event is a festival event, for instance, it should look like this in your Festival Calendar: