G1 Max Pb= 1.5% C90500 Max Pb= 0.3%
G1 max Sn= 10.5% C90500 max Sn= 11%
G1 minimum Sn= 9.7% C90500 minimum Sn= 9%
They sure don't look exactly the same to me but my reference for C90500 may be different than yours.
I got the C90500 specs from
This reference
If the G1 spec for lead is indeed 1.5% (max) then the two alloys are not the same.