Yes, the Ordnance G numbers are part of the SNL system for parts lists. As Chris posted, G is for vehicles, C is for small and medium artillery, D is for heavy artillery, A is for machine guns etc etc. but these are not the part number on the drawing or the part itself, just the catalogue number of that vehicle or whatever. In some cases, the SNL number does share the same part number as the ordnance number, just with the G104 or G503 prefix for example.
I think in the case of your part, it's nothing to do with that, the number is just the part number prefixed C.