The satellite GPS signals are incredibly weak.

Individual GPS signals typically arrive at the antenna below the receiver’s thermal noise floor and the system relies on processing gain in the receiver to generate a fix.

Depends on the source and what you're trying to exercise. M-Code and spot beams are supposed to provide something like 20 dB of jamming resistance, and if your jammer is not right next to the target then the jamming could be a factor a long way off. (For reference, a target 40 miles from a jammer would see ~20 dB more interference than a target 400 miles away. White Sands Missile Range is 3200 square miles, but much longer north-south than east-west.)