To engage in varying degress of sexual activity, anywhere from kissing or light groping (a little gross,) to major all anal action (really gross.)
Ah man, I really wanna get gross with her tonight.
Getting gross is when you or a group of people make somewhere gross for example normally i get gross when i spray fart spray at resturaunts. Normally getting gross is just making somewhere smell like feces but there are many ways like letting rats out during a movie.
Ronny and Greg wanted to get gross ass fuck so they sprayed liquid ass in a Taco Bell the smell was so gross people had to evacutae. Now they are addicted to Getting Gross.