This is an (unfinished, but very far along) implementation of redirectable gravity for the Source Engine. I have no intent of finishing it since switching to Linux and there being no port of Alien Swarm for Linux. To my knowledge, it functions as expected apart from the camera rotation (never finished) and some camera jitter when walking on walls (possibly a prediction/interpolation issue). The player model is also not rotated.
If someone were to port this to Source 2013 in a similarly functional state I'd probably finish it.