Ah, I presume you mean 'The Keeper of the Seventh Gate' then... which seems to refer to the gate to the Seventh Hell, the deepest part of the.. um.. Seven Hells of Hydaelyn's underworld. Incidentally, a boss battle track from the version 1.0 soundtrack (by Nobuo Uematsu) is called The Seventh Gate (it was used mostly for solo quest battles), and in the lyrics for Titan's theme Under the Weight, you can hear Ferne shouting (amongst other things), "I'll drag you right down to the Seventh Gate!" ;)
Although nothing is known for certainty, if I was a betting cat, I'd say 'Seventh Gate'/'Seventh Hell' is basically an Eorzean euphemism for Hell/Limbo/generic place of evil used when cussing, which would definitely make sense in the context of Cid's exclamation. ;)
