1) Not humans, Banks just calls them that in the text of the books, and 2) Any mind derived from a "human like" (even to a very small degree, really human like civilization and evolved) is at a massive disadvantage to a very highly optimized result of recursive self improvement.

Actual earth humans are in the books and noted to be of the same general body plan. Close enough.

The Gzilt minds are specifically compared to culture minds and deemed to be comparable in capabilities.

> The Gzilt minds are specifically compared to culture minds and deemed to be comparable in capabilities.

Well, that's not really realistic. Maybe they are only pretending to be based on organics, but really aren't, and just put up a facade.

They're not based on organics, but that's missing the larger point that realism isn't the point of sci fi like the culture series. It's an imaginary setting the author is using to look at society.