Normally there shouldn't be a gap between the rim and the alu cap. On my 993 C4S the caps fitted perfectly, they sank perfectly in the valvehole of the rim, the black stems had just the perfect lenght.
I don't know what the new rims (19" Carrera S) will give, but I HOPE it will be the same perfect match.
The valvecaps are made of aluminium, that's for sure, but I think (not sure though, could be Real Alu too) that the gas cap is plastic with a PERFECT Alu Look Paint, it is QUITE NICE though IMO