Gortash symbolises humanity. Bear with me.
He is a researcher, a scientist, an inventor. He is progress personified. He constructed the Steelwatch from nothing, and he discarded all ethics to do so. And while vile, that is something that has been done a lot in the past. Humans have ever since crossed the line in the name of research and progress. They have done unspeakable things, all in the name of 'advancement' and knowledge. Especially those who had certain degrees of power or in not exceptionally peaceful times, ethics are the first thing that is discarded to conduct research properly. And that is precisely what Gortash does. He ranked his desire for progress higher than personal attachments or sentimentality, and in its name, he's done unspeakably sadistic things in his 'lair' like countless did before him.
But that's not the only thing. Gortash is a tyrant, a conqueror. He invaded places people already existed in and claimed these places, their treasures and their people for himself—something humans love to do and still do to this day. And he was also an arms dealer. The thing that allowed humans to suppress even those physically stronger and more advanced was weaponry. The thing that allowed progression to this degree is where his roots lie.
He paid attention in his history lessons. He made sure to learn from Sarevok's failures. Perhaps even the shortcomings of Bane's old chosen. But even despite, or perhaps because he knew that history, he made the mistake of repeating it. He was so focused on not repeating Sarevok's mistakes that he forgot the other downfalls others have had to experience. Exactly how humanity continues to learn of its own history but always gets focused on specific parts, and as such, he forgot some others and was bound to repeat them just like we're currently repeating mistakes that have happened before.
There is also Gortash's dismissal of the Netherbrain. His 'how bad could it be'-stance. He dismissed a force of nature, dismissed natural evolution because he thought himself above it, kept silent about it, and as such allowed a plague to spread through the sword coast. Something that has happened repeatedly in our past. The last time wasn't even that long ago, if we're honest.
Even the god he serves, his patron, is the only 'humane' one out of the dead three. Bhaal is a force of nature. He is death itself. Myrkul is almost an eldritch being. He's the explanation for what happens after death, and how to defeat the unknown. But Bane is human. His domain is human. Conquest, rulership, tyranny, worship. Those are human things. These are desires only humans have in that way.
But most importantly, he reigned in and controlled the weapon that is Durge. After their murder spree, and after their worship and temple management, he was the one who stilled Durge's hand. Who reminded them of their own humanity. Who insisted they were human. He became their ties to humanity to them.
So, Gortash symbolises humanity, but simply all it's flaws. He's the brutality, the ruthlessness, the endless desire and strive for progress. The desperation to be 'more'. The cruelty we display in the name of power and knowledge. The sheer lack of compassion, empathy and humanity that only humans are capable of.
And now, this also makes it really funny that he needs to die for you and your companions to succeed. It's almost like you need to kill off that cruelty in a last act of brutality to embrace a 'peaceful' future.
Expect if you're durge cuz you're fucked regardless but that's a diff topic.
