You can cancel a booking from the gig in the app. You'll be asked to pick a reason and can add a note; both go to the agency.
There's no cancellation fee, in either direction. If a gig doesn't go ahead, the fee for it isn't owed. GigSpace doesn't charge you and doesn't charge the venue.
That doesn't make late cancellation harmless. The closer to the date, the less chance the agency has of finding someone, and a venue left short remembers it. Repeated late cancellation is a fair reason for an agency to take you off its roster.
The single most useful thing you can do is cancel as early as you know. An agency with four days' notice will usually fill it. One with four hours often can't.
If you cancel, the agency may offer the gig to a backup act. If a replacement takes it, the fee goes to them.