How can I stop this GFCI receptacle from tripping?
A GFCI outlet works by measuring the magic electrons in and out on the hot and neutral legs and if they are different by a certain amount, trips the circuit to stop electricity from flowing, on the principle that those electrons are going somewhere bad (your body, perhaps).

If the normal outlet is connected to the LOAD side of the GFCI (which it sounds like it is due to your problem statement), then your problem domain is the GFCI outlet, the cabling from the GFCI to the normal outlet, the normal outlet, and the freezer and refrigerator.  As a first step, try unplugging the freezer and the refrigerator to see what happens without them plugged in.  

However, more than likely, the compressor motors on one or the other are at fault, and in general terms, these devices should not be on GFCIs.