The 20-year-old model and the 30-year-old actor first stepped out publicly as a couple in March Kaia Gerber and Austin Butler can’t help falling in, and showing off, their love! The 20-year-old model and the 30-year-old actor shared a kiss on Monday at the Cannes Film Festival, after attending the premiere of Baz Luhrmann’s Elvis. The highly-anticipated film stars Butler …