Vince - since around the mid to late 1960's there there have been at least two variants of this pistol, from some reason rarely found in .177. But first, let's see if we are singing from the same hymn-sheet first.
Original Model 6 - One-piece, futuristic brown or light grey plastic grip, ribbed tunnel over foresight to aid in cocking - came in a hard foam lower box with a clear lid.
Original Model 6M - Recoilless version - identified by the two knobs - one sticking out from each side of the mainspring housing about half-way along. Same grip as the non-recoiling version. Same box.
If this is the one you have, then you have a real problem ahead of you. The Giss recoilless system is a RPITA to fix, let alone put back together after disassembly.
Airgun World magazine did a refurb of the later model 10, which uses the same Giss system, about three years ago, ISTR.
Apart from that, I'd get in touch with Laurie Amatruda - he is a real guru par excellence where these complex air pistols are concerned - the piston rebuild and total overhaul job he did on my Walther LP3 was not only of top-class standard, but cost me less than ten tins of match pellets, too.
Hope this helps
tac