If baby's weight is corresponding to birth weight, has doubled by six months, is steadily increasing, is active and reached some milestones like holding head steady, rolling over,etc. then don't worry... your baby is doing great...

Are you breastfeeding or giving formula to baby? breastfed babies dont gain much weight, but their emotional, mental development and immunity  is way to stronger. Second, weight gain numbers also depends on birth weight. what was his birth weight?