Sorry, you're right and I wrong... never post early in the morning...
I don't have a close-up of starboard rear-fuselage of a G-14/AS at hand, will post if I find it.
At any rate, the hatch you are referring to, IIRC was only for compressed air if a Mk108 was fitted, and those Gustavs didn't have that weapon as standard, AFAIK.
A lot of technical drawings are still incorrectly reporting the hatch on almost all G-versions and I remember also John Beaman admitting that his drawings too were wrong in this respect saying that (I quote) "...The circular hatch on the rear fuselage is an access point for the cocking mechanism for the Mk 108 in aircraft so equipped."
HTH