You need to play for an hour without pausing, exiting the game mode to the menu, or obviously exiting out of the game. Either complete the requirement naturally via an intense Gravity Guy session, or let your character continuously die for one hour. Some Windows Phones have the option to prevent screen-time out entirely (Settings > Lock+Wallpaper > Screen times out after > Never), but most phones only allow up to 5 minutes (presumably to prevent burning out the screens). Set your screen timeout to 5 minutes and every couple of minutes just flip gravity to prevent the screen from locking out. If you are lucky enough to be able to set your screen timeout to "Never," just let it sit for the full hour.