The HC12 is a very inexpensive, long-ish range serial transmitter that is pretty well suited to relaying GPS data at its normal rate of around one sentence a second. I have achieved well over 800m range with it.
There are some rough notes here, and plenty more around the Web for Arduino which you can adapt. It's actually cheap enough just to buy a couple and try them out. If the range is not quite enough, you could maybe add an additional HC12 at half the distance to act as a relay/repeater controlled by a £4 Raspberry Pi Pico or Adafruit QT Py and operating on two different channels of which the HC12 has around 100 IIRC.
Failing that, you're probably looking at LoRa but that will be 5x more expensive. See Andreas Spiess on YouTube for a good grounding in that technology.