Guillaume is in a long-term-relationship with French actress Alysson Paradis, the sister of actress and singer Vanessa Paradis. In April 2015, Alysson announced on social media that she and Guillaume are expecting their first child. On September 1, 2015, the two became parents of a little boy.
Guillaume and Alysson are both supporters of the M Foundation (for women an children rights all over the world).