No, you cannot change the codes or the amount in box 2a. Code G by itself would be for a distribution from your own qualified retirement plan. For this distribution from your deceased wife's TSP account, the codes must be G *and* 4. Because you rolled it over to a Roth IRA, the entire amount rolled over is taxable to you. Had you rolled it to a traditional IRA instead, taxes would have continued to be deferred (and box 2a would have been zero).
If you don't like the tax result, you can recharacterize some or all of this taxable rollover to be a rollover to a traditional IRA instead, thereby continuing to defer taxes and allowing you to convert to Roth in smaller amounts over multiple years. If you want to recharacterize any of this taxable rollover, you must contact the Roth IRA custodian to initiate that.
