If your Form W-2 has a Code G in Box 12, this indicates elective deferrals and employer contributions (including nonelective deferrals) in your section 457(b) deferred compensation plan. There is no deduction for your elective deferral contributions as they are pre-tax.
According to IRS
Retirement Topics - Contributions, if your plan allows after-tax contributions they are not excluded from your income and you cannot deduct them on your tax return.
See also: IRS
Retirement Topics - 457(b) Contribution Limits.
