Perhaps for the gravity options you could bind only one key, which would display a pop-up menu, that contains all of the gravity options, upon being pressed?
Laptops/low-keycount-keyboards almost all have a key/key combo to enable a number pad over 789 down. A percentage of user clicking a button then using the keys is less of an inconvenience than all users using a button then clicking (in my opinion) though I do have an idea...
Someone somewhere on the forums suggested any direction gravity and I said I'd think about it and I have, what if you pressed "g" and clicked (like the line tool) and the direction you dragged was the gravity, all directions any angle with the length being the strength? Using basic math I can code in the ability without too much time really so I'll try it when I get home.