The GEMINIs were in essence copies of two advanced droids: SCORPIO and GEMINI Prime.The latter of these needed to act as a template, as fabricating GEMINI droids wasn't easy; SCORPIO stated that this was because her technology wasn't easy to duplicate.
Three thousand years before the Galactic War, an unknown race constructed GEMINI Prime, based off the technology of SCORPIO, and through the Prime, they constructed an unknown number of copies to act as captains of the Eternal Fleet.
The Eternal Fleet conquered nearly all of Wild Space, before the Gravestone deactivated the fleet and forced it into dormancy.
As the years passed, the Alliance eventually recruited SCORPIO, or the Lady of Sorrows as she was known as at the time.
During SCORPIO's time opposing the Eternal Throne, she had begun to research the GEMINI units, leading to various discoveries such as the Hyperwave Relay Station, or GEMINI Prime.
An order issued from the Eternal Throne was not sent to an individual droid, but to the hive mind for assignment to the best-suited unit.
While each droid retained a limited sense of individuality, she acted more like an appendage of a singular brain than an individual connected to a network.
Their existence was relatively unknown, even within the Eternal Empire.
SCORPIO was investigating them during her time on Zakuul, and Arcann occasionally received reports from the GEMINIs via holo rather than text-based communication.
Subsequently, the Eternal Fleet battled against the Alliance fleet again, which had responded to Koth Vortena's distress call.