HF compresser pops gfci on different circuit
Neutral or ground for the compressor circuit is somehow tangled up with neutral or ground from the freezer circuit.  Go through both circuits and make sure of several things: 


the neutrals from both circuits are totally separated.  No contact whatsoever. 
on both circuits, Neutral is not contacting ground anywhere.  
the ground wires for each circuit goes all the way back to the panel. 
there are no "islanded grounds", i.e. Where some devices are grounded only to each other but there is no grounding path back to the panel.  Sometimes when people add wiring to old pre-ground circuits, there's a ground wire in the cable so they hook it up.  Never do that unless there is a ground path back to the panel.  This ground can be retrofit if need be. 


Keep in mind ground paths can be things other than wires, e.g. If the compressor is physically touching the freezer.