launch/start an App (Activity) by a gesture on the homescreen
You can install a third-party homescreen (launcher) app which supports gestures. Some examples: Holo Launcher HD, Apex Launcher, Nova Launcher. Note that such gestures will work only when the launcher is active (i.e., they are not global gestures accessible from any app), and may be eaten by scrollable widgets if they are present at the homescreen, and you start the gesture inside such widget.

Globally accessible gestures are possible too (e.g., there are apps such as SwipePad), however, this will increase resource consumption, because such app must be running all the time and listening for touch events, and also may conflict with other apps.