How do I stop my toilet from backing up?
Sounds like you have a slow-draining blockage in the drain arm between the toilet and the main drain.  If you haven't used it in a while, the drain is empty and you can flush it, filling the arm with water and forcing the air back out of the toilet (hence the gurgle).  The second time or third time you flush, the drain is already full and there's no place for the water to go.

Call a plumber to snake it and clear the blockage.