The word used is actually charisma. If anything, undeserved mercy is a closer translation. As such, I am not sure it is unconditional. God pardons us, but with a caveat.
While I did not find the two words "free gift" together so much, I did find the word "gift" all over the place in the Douay-Rheims, and in the correct context. For example:
Eph. 2:8 : 8 For by grace you are saved through faith, and that not of yourselves, for it is the gift of God; ... DR
So, if gift means free gift, and I think we agree that it does, then how could this be an invention of the KJV?
The redundancy is the invention of the KJV. A gift is only a gift if it is free, even Dr. E agrees with me on that one (miracle!). :)
There is no NEED to specify it as 'free' as it would imply that there is such a thing as non-free gift. Verbosity comes to mind.
But either way, the word used in Greek is "charisma." If that's what God inspired the Apostles to use, then we should not change them, should we?