Each venue can have an approved backup list: acts that venue is happy to have at short notice. You keep it in order of preference.
When a booked act cancels, GigSpace offers the gig to those backups. Depending on your setting it either offers to all of them at once with the first to accept winning, or lets you pick one by hand.
Under 24 hours out, a cancellation is treated as urgent and moves faster.
If no backup accepts, the gig is marked unfilled and you're alerted: it stays open rather than quietly disappearing, because an unfilled Saturday is something you need to see.
A per-gig backup list is taken from the venue's pool when the gig is created, and you can adjust it for that gig alone.