GPS signal jammer will jam drone's operation

So this would disable communications with the drone, but all other systems should remain functioning?

If the drone had appropriate programming to recognize a jamming scenario and a GPS signal on its launch position, it could presumably react and go back home?

It's very difficult to maintain wide spectrum RF jamming across a large area without an extremely large system that requires a large amount of power. It's more economical to target certain frequencies that you might know an enemy would use, but that only works if they use those range of frequencies.

An advanced capability evolution of the Raytheon ADM-160C Miniature Air-Launched Decoy – cell phone signal jammer has successfully flown a series of free flight demonstrations on the US Navy’s (USN’s) Point Mugu Sea Range off California.

Known as MALD-X, the initiative is intended to demonstrate capabilities potentially applicable to the USN’s planned MALD-N stand-in jammer, and at the same time establish an upgrade path for the US Air Force’s (USAF’s) existing MALD-J inventory. Flight demonstrations were completed on 20 August and 22 August on the Naval Air Warfare Center’s Point Mugu range.

GPS drone jammer directional jamming principle

Duke began life as a low-power, short-range jammer to keep radio-controlled roadside bombs from going off, but has evolved into a much more sophisticated system capable of detecting and disrupting a wide range of signals, from cell phones to the control links for enemy drones.

Those antennas would be harder to jam due to directionality, but you'd have to consider the difference in power outputs too. Is a 50W 4G jammer 10km away going to trump an antenna transmitting/receiving a 0.1W signal from 1000km away?

A handheld drone jammer will be putting out jack relative to a land based omnidirectional antenna, as the Omni direction antenna will have access to far more power and can get hotter before being inoperable.