Somebody that seems to be obsessed, addicted or mentally dependable, or just really enjoys always being and working at the gym. They are usually quite built and packed full of supplements and protein shakes.
Gym Junkies generally fall into a very depressive state if they do not get their daily dose of gym workout.
Person 1: Gym Junkies are faggots they have no life
Gym Junkie: You die now
Person 1: What do you mea - *CRACK*
*Person 1 sails 200 miles into the air with a large dent in his face. He hits the ground and dies from head trauma*