I have dealt with a lot of guys like you that say they can not gain weight. The first thing you need to do is start a diet log, write down everything you eat and add up the cals and macros. Once you see how much you typically eat it will give you a base line for increasing, use the BMR calculator and compare the numbers, Try eating for your goal weight plus10 so if you want to be 175lbs eat for a 185lber, if you still don't gain, eat more. If you feel like your gaining to much fat go back (doubt that will happen). If you are dedicated to eating, and it will be hard, you will see results both on the scale and in the gym. Really you'll probably want to start at around 3000cals, and I would not do low carbs.
99% of the time I ask how much do you eat, answer-a lot, reality they are barely hanging on to what they have. Log, Log, Log!!!