Similar to the French words mec (M.) and meuf (F.), this pair, gros and grosse, mean dude. It's very informal, it can only be used with friends, and the literal meaning, fat, is not relevant at all here.
"Salut gros, ça fait longtemps ! Ça va toi?"
"bah ouais grosse ! J'étais en vacances dans le sud et toi?"
"Hi fat, it's been a while! How are you doing?"
"yeah fat ! I was in the south for the holiday and you?"