Scratch

gravity is a constant "acceleration" - that means changing your speed (not your position) by the same amount every frame, then changing your position by speed. So you WILL need a variable.

when gf clicked
set [y speed v] to [0]
forever
change y by (y speed)
if <touching [ground sprite v]?>
   set [y speed v] to [0]
   if <key [up arrow v] pressed?>
      set [y speed v] to [20]
      end
else
   change [y speed v] by [-3]
end

This is with a constant acceleration of -3 pixels per frame, which is what i usually use.