The GDL-82 ADS-B Out device does not have Bluetooth connectivity so it can not directly connect and send data to ForeFlight Mobile. Provided the GDL-82 is correctly configured at installation* the GDL82 will work well in combination with a dual-band ADS-B receiver such as a Sentry, Sentry Mini, or Stratus 3, 2S, or 2.
The GDL-82 provides ADS-B Out, while the Sentry or other ADS-B receivers will let you view ADS-B In data (weather & traffic) in ForeFlight Mobile. The GDL-82 may also be used with a single-band ADS-B receiver (which can only receive the UAT 978 band) though a dual-band receiver is preferred because they can receive both traffic bands: UAT (978) and 1090ES.
*Both "1090 ES Receive Capable:" and "UAT Receive Capable:" should be set to "Capable" using the Garmin GDL 8X Install Tool (PC-based software). Refer to the Garmin GDL82 STC Installation Manual available here for details.