Benjamin Mayo, 9to5Mac:
The idea is that the watch and the paired gym machine have better knowledge about different aspects of the workout. GymKit allows that data to be shared back and forth, as appropriate.
At the end of the session, the watch saves all the relevant data to its local database (no iPhone required) and the gym equipment is wiped of any personal data.
Walk up to a GymKit compatible machine, tap your Apple Watch (pairs via NFC, similar to Apple Pay), and the machine automatically loads the appropriate workout data from your watch. When your workout is finished, your results are saved back to your Apple Watch.
This is brilliant.