Will the Garmin GNX 375 navigator work with ForeFlight?

Yes, ForeFlight works with the GNX 375. You can flight plan on ForeFlight (or Garmin’s Pilot app) and transfer the flight plan to the GNX 375 via Bluetooth once in the cockpit. While in flight, it’s possible to send flight plans from the GNX 375 to the ForeFlight app, as well as to send amended flight plans from ForeFlight back to the GNX 375.