The best way to defeat a Jammer is to understand how they work.
99% of them jam the 2.4 and 5.8Ghz frequencies.
https://desc-boost.com/blogs/anti-drone-jammer/cover-all-answers-about-drone-jammers
So don’t fly using those frequencies would be a good start.
You can build drones to fly using 1.3Mhz, 72Mhz and 27Mhz.
About the GPS, you can find a more efective distance sensor to get more high without GPS.