The G part is the new Cypress part built on 65 nm node technology while the D part was on 90 nm technology. The pin out is the same for both the parts and hence the G part will be a drop in replacement for the D part. Also the G part has embedded ECC on the device so the reliability will be better for these parts less than 0.1 FIT/Mbit. The operation current will be less for the new part. You can see in the datasheet for both the parts that the functionality is the same for both the parts and there is no change in the AC switching characteristics and slight changes in the DC char.
thank you，It's very helpful！