Propjet88
Jammers work by exploiting the difference in signal received by a receiver from a nearby and remote transmitter.
Since the receiver sees signal levels essentially related to distance squared from the transmitter, it means that a fairly low powered jammer can prevent the receiver hearing a more powerful transmitter at a greater distance.
What is more difficult is when you need to jam a significant bandwidth, say a whole cellular band, with a noise-like signal. The power needed is then integrated over that bandwidth so the design of the jammer needs to allow for that fact.
Since GB and co were about 1km away at the time, it would have taken a stupendous output power to affect the 777 from there, such a power level would probably be enough to disable everything electronic local to them and possibly the humans too.