I've tried that when the player is overlapping the ladder and presses up or down, set the gravity to 0, then move the player up or down TimeDelta*200, and restore gravity if left, right, or jump is pressed...
By the way, I'm using the Platform Behavior.
When I keep going down the ladder and get to the ground, the player starts sliding to the left.
Also, if I press up and THEN jump to the ladder, the gravity is set to 0 anyways.