Up is Y but you are fighting the gameplayercontrol.lua script which is trying to keep the player on the ground (it is a ground based FPS after all!).
You also have the issue of the physics on the player capsule which is also applying gravity so unless you take over control of the player position yourself and reposition every frame (and apply your own physics calculations within that positioning) you are fighting a battle you are unlikely to win.
Start by going through the gameplayercontrol.lua script and figuring out all the places it does any player positioning logic, figure out what it is doing and either comment it out or figure out how to override it externally.
Been there, done that, got all the T-Shirts!