A "Gary" is an insecure man who does not have much going for him so instead he harassed and trolls women online. This definition originated from the largest male troll to every harass female strength athletes who happened to have this name. Since then, this type of behavior has its perpetrators labeled as such a Gary.
*random man* you're too muscular you look like a man Me: okay Gary
Gary is the type of person to try to act cool, but is truly a complete and utter retard. He is typically the type of guy to get at least one divorce in his life and then remarry an uglier girl.