Help: switching Gravity disables Movement

Hello,
i am using a Daydream Controller and the FirstPersonTunnelingLocomotion.cs script from the "DaydreamElements" asset in my game. At one point i switch the gravity with physics.gravity *= -1; in order to walk on the ceiling. My problem is that the movement does not work properly after the switch. The rotation is fine, but the translation does not work at all. When i switch the gravity again, it works perfectly. I disabled the FIrstPersonTunnelingLocomotion and tried the FirstPersonController.cs, which worked fine even after the switch. Has someone an idea why the movement behaves like that?