Where do I add my traditional 457 contributions in? There is a number on my W2 Box 12b G but do I need to add it again?

Your contributions to your 457 plan have already been deducted from your taxable wages reported in box 1 of your form W-2. They are reported also in box 12.

You do not need to report them again in your return.

What if the pre-tax contributions to a 457 plan are not also reported in box 12? Then may they be deducted?

Please check if taxable wages reported in box 1 of form W-2 is less than box 5 by the the amount of your contributions. If so, your contributions have already been excluded from taxation and you cannot deduct them again.