(And note that for gravity to accelerate *any* mass, you have to lift that mass in the opposite direction first.
The more I think about it, the more important I think this point is.
Gravity is *not* a free energy source
Think of these two examples.
Put someone on the ground, climp up a four foot ladder, and drop on them. Do it again, and again, and again. Do it ten times.
Now, put someone up against a wall, stand four feet from them, and slam into them, again..ten times.
In both cases, the same amount of mass is hitting the target. In the end, which is more efficient? In the first case, you have to expend energy to overcome gravity to get up from the ground and onto the ladder. In the second case, you are not directly working against gravity so you do not have to expend much energy to reset your position. Hint: try jumping four feet horizontally...try jumping four feet vertically. The real question is one of accelerationn. Can gravity accelerate your mass fast enough to provide more force into the strike than a horizontal jump. Would I 'hit the ground' falling harder than I 'hit the wall' moving horizontally?
What gravity gives in terms of acceleration, gravity takes in terms of energy it takes to put your body in position to take advantage of that acceleration. So as Arnisador mentions, you can 'raise up' your body to get more mass into play, but it's a zero sum game because it costs you the same energy your getting. May as well just use that energy for a forward strike.