Gil Sloane is Stephen Holder's Narcotics Anonymous sponsor. Sarah Linden first encounters Gil when she witnesses him exchanging envelopes with Holder early on in the investigation. Later, she trails Holder to a NA meeting and sees Holder thank Gil for being his sponsor.
Gil was also Holder's former supervisor in the county sheriff's department, where Gil worked as a narc for 20 years. Gil got Holder assigned to the Homicide division, and later gave Holder the fake incriminating security camera photo of Richmond driving through the toll booth on Desolation Bridge. He later obstructs Sarah Linden’s police investigation by hiding Rosie Larsen’s case files in his storage locker.