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. They are roughly equivalent, technologically, to the Culture.
The Gzilt are pan-human (humanoid), but not mammalian.
The Gzilt have decided to Sublime, having had a referendum on the subject and voted in favour of it. The Gzilt civilisation is organised ostensibly on military lines, with every member apparently serving in the military forces at some stage in some way, and holding a lifelong reserve position afterwards. The Gzilt military is organised along regimental lines, with each person tending to retain some form of loyalty or ties to the Regiment in which they served, at the least because they can be called back out of reserve status if required by that Regiment.
Within the novel The Hydrogen Sonata it is stated that several things tend to happen once a civilisation has made the decision to Sublime. Strange objects called Presences appear, Scavenger species (less technologically advanced species wishing to claim the departing species' technology) arrive, and any other civilisation that has kept anything quiet with regard to the Subliming civilisation customarily reveals it to them before they depart – a setting straight of the record for advanced civilisations.
One such species are called the Zihdren.
Sources: The Hydrogen Sonata