I agree with Martian Destiny that it's the grade much more than the total elevation gain.

I disagree that mapmyride is a good place to find it though. They use VERY wide contour lines that wind up smoothing out most of the climbs in the steepest (and IMO very tough) terrain.

Ridewithgps.com approximates fairly well the elevation gain that I get from my Garmin.

Best way to measure a ride for yourself is to get a computer with a barometric altimeter, or a riding partner who has one. There are some models without GPS that are pretty reasonably priced.