Short answer: yes. Delete the event and the time opens back up on its own.
Why it just works
DrezSync does not keep its own list of what is booked. Every time a visitor picks a date, it asks your Google Calendar what is on it and hides the times that are taken. Your calendar is the record, not a copy of it.
So if you delete an event, move it, or shorten it, the times it was blocking free up by themselves. There is nothing to re-sync and nothing to clear on your side.
How long it takes
Availability is cached for about a minute so repeated clicks stay fast. Delete the event, give it a moment, then reload the form. If it takes longer than that, you are probably looking at a cached page rather than cached availability. See Changes Are Not Showing Up.
The check that runs when someone actually submits is not cached, so this short display cache can never cause a double booking.
What deleting does not do
Deleting the calendar event does not delete the Gravity Forms entry. That entry is your record of the submission and stays under Forms, then Entries. If you want a booking gone from both places, remove it in both.
It also does not tell the customer anything. If they received a calendar invite, deleting the event will notify them through Google. If they only got a form notification email, they hear nothing, so contact them yourself.
The reverse is also true
Because any event on the calendar blocks the times it covers, adding one is how you close a day. That is exactly how you handle a holiday or a day off. See Blocking a Day Off, a Holiday, or a Vacation.