If you give your body enough protein throughout the day, your body won't look to your muscle for fuel. To make sure you are getting enough protein, double your lean body mass in protein grams. If you weigh 80 kg and your body fat percentage is 15 per cent, your lean body mass should be 68 kg. To sustain your present mass, you must consume at least eat 170 grams of protein a day. Please seek the advice of a competent nutritionist to measure your body fat percentage.
Calories also play a very important part in gaining weight.
Generally, a gain in strength coincides with a gain in weight. That is to say that the more you weigh, the more weight you can push. The more weight you can push the stronger you are, and the stronger you are the more muscle you gain. So your strength will follow your weight. This is one area where calories can help you.