: The GM telephone that Cortana used to speak to the Chief was definitely FTL.
: She hacked GM's comms.

Given the bits of Halsey's journal about trying to stick an AI into slispace, and the the fact despite being destroyed or heavily damaged the Gravemind persisted with memories of its incarnation during the Forerunner-Flood War, I think it's pretty much a given that the Gravemind's consciousness resides in some form of slipspace.