Technocube, I would say that every action, available in premium digital pianos, uses gravity to return the hammer. The difference lies in the construction of these actions, here we can see that different manufacturers use different solutions. Mainly one can say that Roland and Yamaha share the same principle, a weight lifted below the key when pressing it down. KAWAI has chosen another technique, a hammer that is thrown away from the key. If you want a good action you should go for KAWAI, Yamaha and Roland have a lot to work on if you ask me.
I believe, it doesnt matter much, if the hammer is above, behind or below the keys.
That doesnt influence the ballistic behaviour.
Once I repaired a Yamaha keyboard with GHE (or was it GHD ?) action and of course this is solely basing on mass inertia and gravity.
But there is one difference to the Kawai:
The yamaha keyboard has the hammer connected to the key with a joint. That means, hammer and key cannot move independently.
If the hammer moves up, the key goes down.
The kawai wooden keyboard is different:
The key has more mass and if you apply a slight stroke to the key, the hammer will "fly" away from the key. There no joint that connects them firmly.
This is -for me- an important difference.
Of course also the hardness or softness of the keybottom and the noise are important. I prefer the Kawai touch. Of course other might differ. It also depends on playing style and habits, if this difference is important.
I dont know, if Yamaha's GH3 works the same way as the GHE/GHD, but I believe it.