Most GPS sets have an active antenna, that is an antenna with a transistor amplifier built in. It is very easy to swamp this amplifier with signals from other equipment and like all transistiors that are overloaded, they switch off, preventing the signal reaching the receiver. Simple solution, move them further apart.
Other problems could be caused by high frequency power supplies, in which case some sort of line filter may cure the problem. A ferrite clamp like they use on computer leads is one option.