The 23-year-old joined the Rugby Park club in April last year but made just three appearances, and was shown a red card in his final outing against Dundee United before leaving the club in January.
He also had a high-profile falling out with the manager Allan Johnston.
Gabriel said: "I took some steps that were wrong but thankfully I got a chance at Waasland Beveren and that has helped me to get over my Kilmarnock nightmare. I am wiser from the experience. I know I deserve a place in the list but my battle with injuries and struggling club career made it a surprise."