I was taught that there are three forces working on a glider - gravity, lift and drag. Since there has to be a force in a given direction to accelerate something in that direction, and since the only force with a downward component is gravity, I therefore struggle with what force is responsible for the downward component of a glider's path (that is a 1g, steady speed glide if it's not gravity.
The forward component of the lift makes it go forwards. The speed is whatever it settles to when the forward component of the lift is balanced by the backward component of the drag. The lower the nose the more forward component the lift has so the faster the speed the glider settles to.
BTW NASA agrees with me about the three forces though for some bizarre reason they rename gravity to weight:
Forces on a Glider
(unless of course your answer is weight rather than gravity - but remember, no gravity, no weight)