As for the gravity, you should set the world gravity. For this, use any object, such as the ball that makes use of Physics behavior. Use its action property to change the world's gravity. You only have to make it for one object.
Like the image below:
[attachment=0:spqssj1i][/attachment:spqssj1i]
If you want an "Earth"'s Physics style, leave it as 10. For the "Moon", try lower values such as 3.