The Contax G stuff is modern, so you'll have threaded filter rings and not any of that series filter stuff -- that's the big pain in the a** in using my old Leicaflexes and lenses, though they're great otherwise. By the time E. Leitz was using 55mm filters they'd moved beyond the series filters and to "normal" filters. And I'm about dead certain that the Contax lens threading will be fully compatible with the Leitz filter.

(Somebody GAVE you Leica SLR equipment? Lucky dog! Anything you, uh, don't need?)