When Asobo decided to make the B350i for MSFS, for some reason they put the wrong avionics into their version of the aircraft. An actual 2020 B350i would have most likely come from the factory with the Rockwell Collin ProLine Fusion TM avionics. Asobo put the Garmin 3X Touch TM in instead. Thus the Garmin, since it is designed for light piston-powered aircraft and not turbine-powered aircraft, displays GPH as opposed to PPH.
Trying to recode the Garmin 3X to a Proline Fusion would be significant work. If you are in contact with Working Title you might want to check with them since they are working on a 3X Touch maybe they can put in a config setting that will allow changing from GPH to PPH.