When I did Washington Pass from Mazama, my Garmin Fenix 3 watch recorded 3,369 feet of elevation gain. According to the best topo maps, the correct value is 3,374 feet.

EDIT: My Edge 800 recorded 3,358 feet. Both are pretty impressive but the watch did a slightly better job than the cyclocomputer. Which is consistent with my experience with both devices. On that particular day, you would expect a barometeric altimeter to slightly underestimate elevation gain due to the temperature.