Gravity Alpha is only functional in newer robots/robot-ware so depending on what you have it might not do anything.
Because of this most people simply use Gravity Beta to tell the robot it is inverted.
Gravity Beta is the robots rotation around the Y axis and is defined with radians. So to tell the robot it is inverted in relation to gravity you can simply put "pi" in gravity beta. 3,14159.
Don't forget to warm start after changing gravity beta.
Cheers!