It is an oxymoron because 'free gift' suggests a possibility that something might be a 'gift' and not be free; it is a redundancy because free is synonymous with a gift; it is incorrect and even misleading because there is no such word in the Greek language original (since some people believe that every word in the Bible is exactly as God ordained it).
I was referring to your reply to my referenced statement, where you tell me that a gift is not a gift unless it is free. On that we agree.
The prof agrees that it probably refers to the church. However, he notes that the church gets its life from the Word of God and sees the church as a support (pillar) for the truth in a subsidiary (subordinate) sense to the word, which is the foundation. So in short, he believes that it is both - God and the Church, as God's subordinate.
"it is a redundancy because free is synonymous with a gift"
That's not necessarily true when it comes to soteriology. Some say before the "gift" is operative it has to be received or accepted or one has to make a decision to receive it or one has to complete the "good works" in order for it to be fully operative. Because of these accretions to the idea of gift, a discriminating term has to be used to explain the simple fact that salvation is by grace alone through faith alone; a gift alone, not of works of any kind or obfuscation.