The phrase "gas giant" was coined by the author James Blish in one of his stories, and is now widely applied to planets such as Jupiter. "The Algebraist" is a novel of gas giants (and the cultures they sustain).
This isn't a Culture novel, but it's the same style of epic space opera that fans of that series have come to enjoy. As with so many of Banks' books, there's easily enough material for three novels. The Archimandrite in particular is villain enough for a book of his own.
Entertaining as the Archimandrite is, this is not a novel about a brutally imaginative sadist. It's a novel in which the power of the seemingly powerless is revealed. It's a satisfying read and highly recommended.