Gregory was elected in 1406 during the riotous Western Schism, when two opposing parties had set up Popes: one in Avignon, another in Rome (the church now considers only the Romans true Popes, so that's who we're drawing here). The cardinals who elected him extracted a promise that if Benedict XIII, the current Avignon pope, would abdicate, then Gregory would abdicate as well - freeing up all the cardinals at once to get together and elect a new pope for the whole Western Church.

Believe it or not, this actually worked - although not the way they planned. It even looked like it might backfire for a while. The cardinals got together several times between 1407 and 1409, asking both popes to come join a council, where they could both peacefully abdicate, without the risk of capture by the other party, or being burned at the stake. Neither pope could overcome these worries however, and at one council in Pisa, the cardinals had had enough.

They declared both popes as heretics, and elected another pope, at Pisa! For six years from 1409 to 1415, there were three reigning popes: Rome, Avignon, and Pisa. Eventually, they all did get together and peacefully abdicate. The cardinals worked out compromises that satisfied all the powerful families with extra church positions, and elected one new Pope to rule over the whole Western Church, ending the schism.

Gregory retired, and spent the remainder of his life out of the spotlight. He died in 1417.