A baby usually gains more than 600 gm per month in first 3 months. So weight gain is little less. It depends on many factors. Height and head circumference are also as important as weight. What was the birth weight? Is the baby on mother's  milk or formula? Please visit a pediatrician for detailed check up

Weight gain can be around 20 to 30gm per day. First 7 to 10 days, child may lose weight to regain it again. So if we consider 50 days, then approx a child can gain 1000 to 1500gm from birth weight. However this are ideal situations. This depends on Birth weight Gestation of child Child illness Feeding difficulties Mother or child's illness However at present if the child is gaining weight 300 to 400 gm in 2 weeks and passing urine 8+ in 24 hours, then continue what you are doing (if exclusively breastfed). Don't add formula milk