Grav shafts are elevators that can direct gravity up and down to allow to quickly change levels.
They consist of an upwards and downwards shaft side by side. The accessible shaft is covered pulsing green lights while the inaccessible one is covered with pulsing red lights and a 'Do Not Enter' sign.
Be aware that Phantoms can also access this Grav Shaft so you may get jumpscared if you met Phantom right in front of you when you exiting Grav Shaft.
Some of the Grav Shaft are locked with codes such as one that will lead you to Executive Lounge on Crew Quarters or Alex Yu's Office on Arboretum. In most of the games, you can either use your GLOO Cannon to make your way up and down or find the code to activate the Grav shaft.