The way I like to trouble shoot a radio that is breaking squelch is to turn on only the radio with no squelch.......wide open, so that it gives all background noise. Then I turn on all other electrical equipment, one at a time. Think of it as filling a dam, noise from each unit adds to the overall water level. When the dam is full it spills over or breaks squelch. All equipment will add a little background noise, but some add tremendous amounts. The Cambridge GPS is about the worst offender I have run across. You can try moving the GPS as far away from radio antenna as possible, do this with radio on as you listen to the background noise in different positions. Best bet may be to get another GPS. Grounding everything to ships ground may help, shielding and ferrit's may also help.
Good luck,
JJ