I believe a standard compatibility screening test looks at about 20ish antigens. I’d google “histocompatibility” or “major histocompatibility complex” for more information.
But back to your original question, unless the gene in question encodes a protein that is expressed on the cell surface (or would have a downstream affect on those proteins), it will more than likely not cause rejection. If it does affect cell surface proteins, you’d have to test to see how much of a reaction it would provoke.