finally I decided to open and dismantle my GH1720qr Gitzo head trying to restore it on my own.
I had to apply a new coat of lubricating solid grease (PTFE type), removing the pristine and sticky one chiefly at the base of the pan mechanism.
I applied this solid grease on the inner parts I got to have access to, whilst I dropped a bit of special lubricating oil on the tilt mechanism.
This treatment resulted in a much smoother pan-tilt movements, giving a perfect and balanced degree of fluidity in both axes.
No more unwanted rigidity and effort when I use the 15x56 Vortex Kaibab on this head.
That's all for the moment!