His Culture novels are a great example of transhumanist SF, and they provide (through the Contact division, and especially its Special Circumstances group) a good example of what interesting things there are to do in a post-human society; namely, mostly, interacting with other, non-post-human societies.
Even more relevant to gaming use is his non-Culture novel Against a Dark Background. It reads like a very well-written actual play report of a group of science fictional mercenaries, perhaps even a Traveller party, who regroup for one last mission. The title does not lie — it is a very dark novel — but it is a wonderful example of worldbuilding. Last time I read it, I couldn’t help but draw maps of the various places in the Thrial system, so evocatively described by Mr. Banks. If someone published it, I’d probably buy some form of reference work to the Thrial system.
And that’s just his work that springs immediately to mind for gaming purposes. Most of his books are wonderful. Check them out if you haven’t.
Iain Banks left a lot of wonderful, challenging, compassionate ideas. He did a lot of good.