I just thought of this all of a sudden. I didn't see a convenient place for a gravity button on the bottom of the window.
Why not simply use the leftover space on the right margin where the element category tabs are? So starting from the top could be Gravity, then Velocity, Pressure, Temperature, Interactions (if you don't want anything reacting.)
It seems to be common in sand simulation games to allow turning off vital things like gravity and interactions. If it isn't feasible to have simple vertical buttons on that margin, then maybe a menu bar that extends from the top margin or the right margin? There ought to be some place to put these things.
(I don't know if adding a boolean check for gravity would slow things down. >_<)
It won't really slow things down much at all :p. Also if need be (especially in the future) the window size can be easily increased without increasing the sandbox size.
uhh if u make a shortcut to powder toy and then in properties under target put "scale:2 with a space between the ".exe" and "scale:2" its a increase in size incase u dont know