If you look at Settings>Privacy & Security>Location Services, you will get an idea of some of the apps that may depend on GPS (not all apps "Location Services" are completely GPS dependent as they can use other things like W-Fi).
For me the most important apps that become disabled or less functional when GPS is switched off is Uber (less convenient to use), camera (no geo tagging of my photos), Pokemon Go (won't work), mapping services (less convenient to use), and weather apps (less convenient to use).