For me I haven't really gained at all. My doctor told me it's fine. He said try to add a bit of weight in the last trimester but as long as the baby is growing and is healthy don't worry about weight gain unless it's a huge concern with your doctor
I've gained about 7 lbs at Week 28. Doctors were happy with it. I'm eating plenty so I'm not too worried about it!
If your doctors are not concerned, and if you had a healthy pregnancy/baby with similar weight gain last time, don't stress! Everyone's body reacts differently to pregnancy. If you're trying to gain more weight, try to focus on protein rich foods that are more calorie dense. Eggs, whole milk yogurts, avocados, dried fruits, bacon, oatmeal, etc. Just Google "weight gain" recipes and you'll find lots of info.
I'm 29 weeks and have only put on 2.5 lbs so far. Baby girl is active and seems to be doing fine. My last Dr. Appointment was about 4 weeks ago and I had only put on half a pound at that time. He didn't seem worried so we will see what he says at my next appointment in a week.