The location of the GPS tracker on your bike mostly depends on the device itself. Some GPS bicycle trackers can fit inside a bottle cage, or hidden in a fender or extra mount. Other devices may stick out a little more, and have to be placed on the bike rather than hidden. Under the saddle is a great location for your GPS tracker, as it is almost impossible to spot unless you are looking specifically at the underside of your saddle.
The inner length of the fork can be a good location as well, just make sure the GPS bicycle tracker isn’t too close to the spokes. You can also put the GPS tracker on the chainstays between the front and rear chainrings, or in the middle of the frame near the front derailleur. Each of these locations are difficult to spot, and even harder to get to for someone who is trying to steal your bike.