The Gzilt are a pan-human species.
The Gzilt are a very old race, being present when the Culture was first being formed. The Gzilt almost joined the Culture, but opted out in the end. Details of this decision play a role in The Hydrogen Sonata. They are roughly equivalent, technologically, to the Culture. Gzilt ships use a virtual crew with accelerated processing instead of a single artificial Mind. Minor conflicts between opposing Culture and Gzilt ships over certain historical artifacts are witnessed before the planned Gzilt subliming.
After Sublimation, a small percentage of Gzilt remained in the Real and are now known as the Gzilt-Remnanter.
The Gzilt are pan-human (humanoid), but not mammalian. They have dark gray skin, which tends to become more reflective with age.
In their medieval stage of development the Gzilt received the Book of Truth from their benefactors the Zihdren. Unlike other holy books, its claims were found to be verified by every scientific breakthrough the Gzilt made over time, and this lead the Gzilt to regard themselves as special or chosen. This attitude and the revelations in the Book of Truth lead them to choose Sublimation sooner than the average for highly-developed civilizations.