If Kos had taken the striker but also kicked the ball out and a right angle so Gabby had zero chance of getting to the ball, its a great tackle. You basically have to see the situation as two events, not just one whole. He slid in, marginally touched the ball... that itself was NOT a foul, but after that(albeit only a split second later) his touch on the ball did NOT take it away from Gabby, Gabby also touched the ball and it was following Gabby's path, the follow through of the tackle took out Gabby and prevented him continuing on with the ball... that is the bit thats the penalty, NOT the slide itself.
They showed a replay from the ref's angle(give or take) and it really doesn't look like Kos touched the ball from that angle, so from what you see from the ref's angle(no touch by Kos, Gabby running, Kos taking him down, ball still going in the path Gabby was running) it was a clear penalty.