If GPS is lost, your drone won't crash or fly off.
It would still be completely controllable, but won't have horizontal position holding ability.
It will hold altitude, but can drift with wind and it won't have any "brakes".
If you encounter GPS...
Your thermal add-on is actually one of the better ones I have ever seen. Using the gimbal is awesome and then getting the new image to overlay on your screen so you still have telemetry info and your RGB feed.
Thumbs up!
(Update)
Made Phantom 3 a thermal drone!
Attached thermal camera and counterweight to the gimbal. Moved vtx and battery on the drone.
Flies great. The gimbal handles the extra weight just fine. Phantom's gyro-stabilized gimbal, which can be...
This is one of the reasons I got a Neo. With a cheap Holy Stone RID module and an Aerial Pixel mount, I have a Category 1 drone. I used it to film some people running the other day. Cool footage, from a perspective I never had before.