Spoilers:
Gail is very much a favorite among the cast, in terms of all of the other characters pretty much liking her and not having some beef with her,
That too.
That third possibility also seems highly unlikely to me, unless Garret's screaming from the agony of his thunderclap on steroids, maybe an illusion caused cast by the entity, had brought Gail, who was close enough to hear, running and the entity pretending to be Conner took the opportunity to attack Gail in an attempt to kill her while also casting an illusion that he's the one who found Garret. Garret, who is being driven insane with pain, snaps and attacks fake Conner and hurts him, losing awareness just as he lunges at Conner, but Conner gets away, and then Garret loses consciousness.
Do you think that the entity who's taken on Conner's appearance, imitating Conner and his mannerisms so well that it's fooled Garret into thinking that it's Conner, was the only one left aboard the Arran before our heroes arrived, or that there are two Conners on board the Arran with the group, the actual Conner who miraclously survived for two years, and an entity pretending to be Conner, as I've mentioned above?