GPS is a passive system.
The satellites in orbit emit a constant location and identifier signal perceivable from the ground.
A GPS positioning device looks for and reads those signals and then performs a triangulation calculation.
Spoofing these signals means outputting identical but incorrect ones either from your own constellation of satellites or from ground-based arrays/emitters.
Typically military hardware expects a certain level of spoofing and has it's own built-in countermeasures/security checks so effective jamming/spoofing means successfully navigating these countermeasures either through brute force or accurate intelligence.
A GPS positioning device looks for and reads those signals and then performs a triangulation calculation.
Spoofing these signals means outputting identical but incorrect ones either from your own constellation of satellites or from ground-based arrays/emitters.
Typically military hardware expects a certain level of spoofing and has it's own built-in countermeasures/security checks so effective jamming/spoofing means successfully navigating these countermeasures either through brute force or accurate intelligence.