Re normal infant weight gain: for the 1st 6 months of life, an infant is suppose to gain at least 600 grams/month then for the next 6 months, it slows down a little so it becomes at least 500 grams/month. So in the 1st year of life, an infant should gain at least 6.6 kgs in addition to the birth weight. In addition to this, there is really some amount of catching up growth that happens in the last 6 months so that at 1 year of age, the child should at least be 10 kgs.
At 1 to 2 y.o., the child's weight should be between 10 to 12 kgs.
Lets say, if you compute it from the birth weight, the child is still in the ideal body weight, but the weight gain is lesser than the expected, then an investigation should be done as to the cause, like lesser milk intake, recent illness, etc. because the child is still considered not gaining adequate weight.
But one other factor to consider is the child's length/height. Actually, it is better to compute the ideal weight based on the height. That is what we call Ideal weight for height. When a child has been underweight for a long time, the height also suffers. So the child ends up thin and small.
Hope this helps...