I had a Diana (marked Gecado) 65 that I purchased new around 1973, and owned it until about 2000. I bought it along with a brand new Diana (Gecado) 35, and I believe that the 35 sold for $150 and the 65 was $300. When I sold it, it needed a set of seals and I was able to get them without difficulty. That was the only service that it saw, or needed, throughout the time that I owned it. The gun functioned perfectly and was very accurate. I distinctly remember the first time that I fired it...the firing behaviour was so smooth and vibration- and recoil-free that I thought something was wrong. I was astounded when I opened it and found that it had indeed fired.
I don't remember that terminology "GISS system"...is that referring to the dual piston recoil-cancelling technology? If so, it works like a charm. I heard at the time that it was considered to be somewhat less durable than the inertial-type recoil-cancelling system used by Feinwerkbau, but in my experience this was a very precise and nicely-made gun. Being a barrel-cocker it was only a small step below the side-lever-cocking Feinwerkbau match rifles. We would regularly use it for shooting flies and hornets at 30 feet with excellent results.
Congratulations on your find...I wish now that I still had mine. I might actually have the owners'manual, and if I locate it you're welcome to it.
John