LAS VEGAS – Kelvin Gastelum doesn’t know what version of Robert Whittaker will step into the octagon when he challenges the UFC middleweight champion later this year. It was announced Friday  that Gastelum (15-3 MMA, 10-3 UFC) will challenge Whittaker (20-4 MMA, 10-3 UFC) for the 185-pound belt after coaching Season 28 of “The Ultimate Fighter” reality series, which premieres Aug.