@Tan The Fuel Gallons (GPH) comes from the JPI Fuel Transducer, I don't believe it has anything to do with any inputs from the GPS.
My shop tried several iterations on JPI and GPS and in now works for me. Here is what I have that works:
JPI - GPS Format: #6: GNS 430/530-WP
Garmin GNX-375 -...