From what I can tell the G-force metre increases when you dont land straight or when you do a really big jump.After playing the beta, I'm on the fence on this game. I've gotten more accustomed to the snowboarding and trick system, and when I was consistently nailing tricks down, it was a very satisfying feeling.
My issue is the game doesn't explain its mechanics very well. I have no idea why sometimes I land after doing a trick my "g-force" meter goes down. I don't know how to move faster in the paragliding challenges or what exactly that beeping meter is. I think I could enjoy a game like this but I wouldn't pay full price for it so far.
The beeping metre is am indication of how close you are to the ground I think. It could also be related to how fast you can rise.