Well, check the Encyclopedia or Memory Alpha for "Gamma 7 sector" or "Gamma 7 outpost". Apparently, this place is close to the Cardassian DMZ and also to the Gagarin Research Station. By the Star Charts assumptions, this would probably indeed mean somewhere near Gamma Ceti, because that's where the DMZ is - and that's not a bad location for Gagarin, either.
Or for a low-numbered, TOS era starbase. Apparently, that route between Klingon and Cardassian territories is a popular one for UFP expansion and trade, since several other TOS era bases are around there. In Kirk's time, with the slower ships of that era, the bases would still be relatively far apart, though, allowing for the plots of the episodes in question.
And as you say, we have SB G-6 in that area, because it's near Betazed which by the Charts assumptions on Dominion War events must be around there. If that's the same place as SB 6 from TOS, why, there you go.
Except that in "Ultimate Computer", SB 6 was Kirk's port of call before he set out to explore the Alpha Carinae / Canopus system, which is a real one and lies in a completely different direction.
...Except that in the original episode, Kirk's port of call wasn't given a name at all. "SB 6" is the invention of the people responsible for the new effects that were created for the remastered version of the episode. So if you don't believe in the remastering, go for the "G-6 is 6" solution. It's not as if even the remastered episode explicitly gave us more than an ambiguous structure with perhaps a blurry "6" painted somewhere...
Timo Saloniemi