As it happens, Gamzee has a line I never gave much weight to before noticing Bro’s SAW interest that I’m more inclined to take seriously now, that suggests Gamzee and Bro’s relationship to Cal WAS intrinsically different:

But even if Bro is kind of a noble captor figure holding Cal back, I still wouldn’t think it excuses any of what he put Dave through. He’s still an awful dude.

As for Gamzee, here’s the main problem with reading him as “just” brainwashed.

Gamzee doesn’t require Lil Cal’s presence to go evil. In fact, Gamzee doesn’t seem to require ANYTHING to turn evil. 
But even if like, Doc Scratch ALWAYS teleports Lil Cal into Gamzee’s presence to trigger his personality shift, I don’t think it would matter.
The weight of the sheer SCALE of Gamzee’s devotion cements his place as an ultimately willing accomplice/acolyte to Caliborn’s Dark Carnival. 

And it kind of makes Gamzee fucking terrifying and a fantastic villain.

I’ll explain my reasoning here.

We know for a fact that Gamzee snaps and kills all his friends in at least one Doomed timeline. This is the source for half the code used in the creation of Doc Scratch. There’s no implication that Lil Cal is involved here at all. 

But again, let’s assume Lil Cal was here again. It doesn’t matter.

Because there is canonically, explicitly, no timeline in the history of Gamzee where Gamzee ever, ever, EVER chooses to rebel. Gamzee Makara simply does not ever choose his friends over Lord English.  In any timeline. Ever.
How do I know?

Lets talk about Ghosts for a minute. The fandom has historically kind of taken these guys for granted, and loose fandom consensus is that they aren’t coherent/who has what ghosts is arbitrary. This is incorrect!

Pretty much everybody in the Bubbles that should have alt!ghosts does, including Meenah and Aranea, the two characters who’s alt!ghosts are typically presumed “Missing”. 

This is important. The Ghosts kind of give us very low-key character development, and contextualize the characters for us. For example, Eridan is an absolute irredeemable bastard in the Alpha timeline. But in a God Tier iteration of themselves, Eridan and Feferi seemingly come to friendlier terms. In another, there’s suggestions Eridan makes up with Feferi and Sollux. In yet another, he seems to be Trans or exploring femininity at least.

The point is, there’s a certain fluidity to Eridan’s potential. Still terrible in the comic, but it’s important to remember that Eridan didn’t CHOOSE to be trapped in the meteor with Jack, or to be born to Alternia’s power system, or to be trapped in the Alpha Timeline. 

It’s important to remember these things because in Homestuck, someone with power–Lord English–deliberately and willfully chose those things FOR him. Eridan’s lives are lived in response to that imposed power structure.
These factors don’t redeem him completely necessarily

But anyway, the fact that the rest of the cast have coherent quantum expressions means there are only three real exceptions–three characters who either don’t have any ghosts at all, or should have more ghosts than they do. 

The first is Caliborn, who’s timeline has exactly one deviation from the Alpha–apparently caused by John’s retcon. This riddle’s solved easily enough:
Predomination doesn’t leave a ghost to appear in the bubbles at all.
When Calliope says she ate his soul, she means that literally. 
Caliborn’s cheating in the Alpha Timeline is indeed the only reason Calliope exists in the bubbles at all.

(This, by the way, explains a lot about the relationship between Caliborn’s soul and Gamzee/Arquis’ in the Lord English. He predominated over them, too.)

The second is Vriska, who only has a single ghost in (Vriska). This is really weird, because we literally know for a fact she dies in more than one doomed timeline! As with the two Calliopes, I think this is down to John’s retcon doing some weird entanglement nonsense to Vriska’s quantum existence.
The point is: Where others have a palette of possibility, Vriska has two extremely polarized halves. Schrodinger’s Vriska. 

Important to mention that just like Eridan, the structure of the Alpha Timeline that limits potential Vriskas is IMPOSED ONTO HER. Vriska didn’t want anything about the way she was raised or where she was born. She didn’t ask John and Terezi to retcon her into this bizarre state. Both Vriskas, like the rest of the cast, are rolling with the punches LE has seen fit to give. 

Except for Gamzee.

Hussie literally tells us Gamzee never dies. His single non-Alpha Timeline death in [S] Game Over is retconned by John, and Hussie suggests it straight up doesn’t count. But that presents a problem.

There are thousands upon thousands of Doomed troll timelines. How is it that Gamzee specifically never ever EVER dies? Well, there’s only one real way that a Non-Time player can survive a Doomed timeline, that we know of:

Dream self merger. By going to sleep as the last player present in Sburb, the Doomed Rose from Davesprite’s timeline triggers a game mechanic that ends her timeline completely and merges her consciousness with that of Alpha Rose through their dreamselves. 

If Gamzee survives his doomed timelines, this is the only possible way how.
And collapsing all of his potential instances into a single Alpha identity certainly sounds like the reduction of possibility commonly attributed to the Rage aspect.
But what that means is that to move on to the Alpha, every Doomed Gamzee must inevitably either snap and kill all the other trolls, or somehow outlast them. 

And it means that if any Gamzee had EVER, in the entire spectrum of plausibility the Alpha timeline affords, EVER been inclined to rebel against LE–then we would know. Because somewhere out there, that at least Hussie could see, there would be a Ghost to show for it.

But there isn’t. Similar in this respect only to Caliborn, Gamzee simply has no alternate deviations because he doesn’t want them. He chooses the path that leads to Lord English freely and willingly, over and over and over again. 

And like Caliborn…

Gamzee does this because he wants to. 

Gamzee doesn’t BELIEVE he’s going to become his own God–he knows it for a fact. He sees it in Lil Cal’s mangled soul. And he embraces that truth wholeheartedly, throwing himself into the acolyte role from then onwards and presumably following instructions Doc Scratch gives him throughout Act 6. 

Which we can talk about some other time. The point is: Gamzee chooses all this. Whether or not Lil Cal causes him to is beside the point, because there is not and never will be any timeline where Gamzee chooses to resist. 

Gamzee is the ultimate in shitty cosmic nazi religious zealots, and devoted to the very power structure that causes every other character to suffer so. There are no mitigating factors for him as there are for everyone else but Caliborn.
At the end of the day, he’s evil.
Bad clown. Worst enemy.