Can ForeFlight and Garmin avionics talk to each other?

Yes. ForeFlight Mobile connects to select Garmin avionics via the Flight Stream 110, 210, and 510, Garmin’s family of Bluetooth wireless gateways, enabling convenient two-way flight plan transfer (Flight Stream 210 and 510 only, requires a compatible Garmin navigator) and the display of subscription-free ADS-B weather and traffic, backup attitude, and GPS position on your iPad and iPhone.

ForeFlight can also connect to Garmin's GTX 345 ADS-B Out transponder to receive GPS, AHRS, pressure altitude, and ADS-B weather and traffic information with select models. This transponder can connect to ForeFlight directly via Bluetooth, and does not require a Flight Stream. You can learn more about ForeFlight Connect for Garmin avionics here, or see this table for a side-by-side comparison of options.