I am an easy gainer (fat and muscle)..........if you are gaining weight, fat is going to be part of that weight.....
Think of a steak.......there is still marbling (bits of fat in the muscle) that give it size...
Humans are no different...as you gain weight, especially with AAS....there will be a portion of fat and water associated with a weight gain.....
the key to minimize fat gain is purely an exercise in proper diet......do some research on lean body mass calculation, basal metabolic rate (adj for exercise)....keep your caloric intake slightly higher than your BMR to keep gains flowing on the lean side....as you gain weight make sure you adjust your intake properly