A soft kiss on the side of a man's scrotum as a greeting or show of appreciation. In some parts of the world, two kisses are given...one on either side (ie, Paris, Barcelona, etc.). A gentleman's kiss has also been used as a form of apology.
Gentleman #1: "Hey man. Sorry I ate the last of your Kashi cereal. Gentleman's kiss?"
Gentleman #2: "Yup."