An oxymoron are two words used together that contradict one another, yet convey a meaning like "Jumbo Shrimp". A redundant expression is one that repeats itself such as "those cars". The term "free gift" is neither an oxymoron or redundant.
If you think a gift is always free, I'd be happy to send you some of my mail where I'll receive my "free gift" if I spend $25 in their store or if I include $9.95 for shipping and handling to receive my "free gift". That is more in line with the Orthodox/Catholic concept of a free gift. The gift is given but you must work for it and earn it. You are required to pay something for it. In the end it's really not free and it's not a gift.
Protestants believe this gift is "free". It's a "(divine) gratuity", "deliverance", "a (spiritual) endowment" to use the Greek dictionary for "free gift". I especially like the term "endowment" because it implies giving something to us which is precisely what God does. He imputes righteousness to us, causings us to walk in his steps. He endows us with certain abilities and attributes that will cause us to walk in His path and be obedient. This is an endowment that He gives to us believers.
As you can see, these are two completely separate models.
A shrimp is not a synonym for smallness, but a name for a marine animal that became a synonym for smallness. Thus, "jumbo shrimp" is not an oxymoron.
"Those cars" is not redundant. It is a prepositionally statement.
But I agree that 'free gift' is a redundancy in a strict sense, although the expression in its entirety is rather moronic. Because there is no such thing as 'non-free gift' it is both redundant and silly to even put 'free' in front of the gift. Affirming something that exists only in affirmative is just as stupid as denying something that doesn't exist (i.e. non-chalant).
Apparently the authors of KJV liked it so much they even put it where the Greek text did not even mention 'charisma.'