How do I prevent my GPS-tracking app from stopping?
The Android OS intentionally tries to prevent applications like this from running to prevent battery drain. For Android 6 specifically, you need to know about the doze and app standby features. Doze prevents applications from running when the device is not in use. App standby defers network activity for seldom-used applications. Both of these are likely affecting your usage. 

Running GPS (actual GPS, not just location services), is a huge power drain. 

If you don't care about power, you can try: 


Keep device on with your application in foreground. 
Whitelist your application for doze and app standby (see above link).