Distributions from a nonqualified 457(b) plan are required to reported by the payer in box 1 and in box 11 of Form W-2. These are included in your AGI for calculating Medicare premiums. The amount shown in box 11 is the amount of box 1 that is not subject to Social Security and Medicare taxes because these taxes were already been paid on this portion when deferred into the 457(b).
Code G in box 12 of a W-2 has nothing to do with reporting distributions from a 457(b). It is for reporting amounts of income that are deferred (contributed) into a 457(b) plan.
