If you have a Navien Unit tripping the GFCI outlet, see below. 

Talked to an electrical engineer at NAVIEN. Any appliance that rectifies through the ground will create an imbalance between the ground and the neutral, and therefore trip a GFCI outlet. Two potential work arounds for for this to still pass inspection:

1) Use a GFCI breaker instead of an outlet.

2) If the unit is piped with bonded black pipe, add a ground from the chassis to the gas pipe