Tony Hawks Project 8

Tony Hawk games always have many gaps that you must perform (and land) at least once throughout your career to unlock the 100% gaps achievement. When you land a gap, it's name will appear at the bottom of the screen in blue, and the gap list will be updated in the menus. A gap can be a ramp, transfer, grind, manual, revert, etc. For a complete list of the gaps in Tony Hawk: Project 8, follow the link below. Thanks goes to Ryan, from the Gamefaqs.com guides.

http://www.gamefaqs.com/xbox360/932714-tony-hawks-project-8/...

Once you find 50%, you'll unlock the achievement.