It's a non issue. As I mentioned earlier, REWARDS is found in both the Tanakh and the NT. While 'free gift' is found only in the NT. For something to be true in the NT is must be supported by the Tanakh.
Your question thus become a non issue. :)
Also I noticed some verse which suggest that the gift is the Holy Ghost and not Salvation.
I promise I am not laughing right now. Honest, I'm not. Ok, a little. But just a bit.