I don’t think Gellar is imaginary. I think that’s what the show writers have gone out of their way to make us believe, perhaps a bit too obviously, in order to set us up for a twist that may not pay off so well.
Unlike The Sixth Sense and Fight Club where the imaginary reveal comes as a surprise, and later more than sooner, Dexter has been sledgehammering us from the get-go with the idea that Gellar is to Travis as Harry is to his son: the shades of dead father-figures who counsel from the great beyond. The problem is that despite the avalanche of clues pointing in this direction, each clue can be rationalized on the assumption that Gellar is real, and there is actually a clue that does indicate that Gellar is real. Let’s consider all the evidence.
In favor of Gellar being imaginary:
* In general, no one ever interacts with Gellar except Travis. This is the prime selling point for the idea that Gellar is like Harry: once a mentor, now dead, but still mentoring inside the pupil’s head.
* In particular, when Travis and Gellar are at a restaurant (episode 4), the waitress pours a drink for Travis and talks to him but completely fails to acknowledge Gellar in any way. On the other hand, the waitress does know Travis, that’s why she’s so chummy with him, and Gellar could have already said that he’s not ordering anything.
* When Travis and Gellar are out in public (episode 5), no one notices Gellar despite the newspapers broadcasting his photo as the Doomsday Killer. On the other hand, Gellar does acknowledge that he should get out of sight, and who pays attention to tabloids anyway?
* Travis abducts victims by himself (the Horseman of the Apocalypse, the Angel of Retribution, the first potential Whore of Babylon), or with Gellar remaining in the car (the snake victim, the second potential Whore of Babylon), but never with Gellar getting his fingers dirty. On the other hand, this is typical of cult leaders who manipulate their followers to take the biggest risks.
* Gellar is inconsistent on the matter of free will. In episode 4 he assures Travis that people have free will, while in episode 7 he disdains the idea, declaring that people’s wills don’t matter. This makes sense if Travis is conflicted about predestination and is having internal arguments with himself. On the other hand, Gellar does not exactly say there is no free will in episode 7, only that free will has no power to stop God’s overarching plans.
* Gellar evades Dexter by escaping from a second-floor window of the church (episode 8). But there could be another way down which we (and Dexter) haven’t seen yet, or Gellar could just be hiding. And we know that Travis is really chained to the floor.
In favor of Gellar being real:
* Gellar spies on Travis through a door crack when Travis is having sex with the angel of death victim (episode 4). Travis is oblivious to this, implying an objective reality on Gellar’s part. Certainly Harry never appears without Dexter’s awareness — that’s the whole point of being inside someone’s head.
I’m nervous about the upcoming reveal that Gellar is real, because we’ve been yanked too strongly in the opposite direction. The result is that, in retrospect, all the scenes of Gellar not interacting with the world seem forced and rather unfair to the viewer.
But if it turns out that Gellar is indeed imaginary, then that’s even worse, for the entire season has been reduced to a banal exercise, when Dexter has always been more reliable about supplying surprising twists. On top of that, the writers haven’t played fair ball: the spying Gellar in Travis’ sex scene implies an objective reality.
UPDATE: Episode 9 makes plain that Gellar is imaginary, that he’s been dead for some time. So we went through all those episodes of the obvious to get to an unsurprising twist, with an unfair scene in episode 4 that implies Gellar is real. I’m nonplussed.