Code G is for both elective and non-elective deferrals.

If it is an elective deferral, on the W-2 worksheet, look to the right of box 12 for the question about code G, and check the elective box.

I'm not very familiar with 457(b) plans, but interestingly if you check the "not a state or local government" box, it takes the credit away.  It seems that other ones may not qualify?