Within the main canon of Star Trek films, TV series and the animated series there are no further mentions of the planet from which Gary Seven emanated (referred to on Memory Alpha as the Planet of the Assigners). The episode in which he appeared was intended to be a backdoor pilot for a spin-off show that ultimately never happened for a variety of reasons and further mentions seem to be have been intentionally avoided.
Note that he states that his "homeworld" is not a human world but is, in fact a cloaked alien world and that he was simply one of a potentially wide variety of species that had been abducted in their cultural infancy and used as forced breeding stock for future generations of covert agents :
SEVEN: Roberta, you've got to believe me. Look, a truly advanced planet wouldn't use force. They wouldn't come here in strange alien
forms. The best of all possible methods would be to take human beings
to their world, train them for generations until they're needed here.
As far as "Extended materials" are concerned, Gary Seven appears over a dozen licensed books and comics as well as numerous fanfic stories.
Although his homeworld remains shrouded in mystery in all of these properties, we can find a short excerpt in The Rise and Fall of Khan Noonien Singh, Vol 1 where Gary almost blabs the location of this cloaked planet:
“Where do you come from?” she cross-examined Seven. “Where?”
She held her breath as the crucial information dribbled out of Seven
with excruciating slowness, one word at a time. “Cloaked planet,
light-yearss away, located in Ssystem Zeta-Gamma-Five-Three-Ssev-“