It is the nickname of a girl that is a friend of a guy that he may think is cute but likes to joke around with, usually a small girl hence the name Gatito (Kitten in Spanish). Any reference to a cat the guy has to call it out to her and give her a little elbow nudge and smile.
Movie: "She is one bad kitty" (Guy nudges girl)
"Hey Gatito, is that 12 year old taller than you???" (giggle giggle)