Not GP, but I guess I'm using the same app. You guess (and then it gives you the price up front). 10 minutes before it expires it asks you if you want to extend it. There might also have been a detect if you drive away and stop feature (don't recall).
Mostly these days all paid parking has registration camera's, and it just starts and stops parking for you automatically. However, there are like 3 or so apps that compete here so you need a profile with all of them for this to work and you also need to enable this on all the apps.