Firstly, there are several such questions a month asked on TQ, and all of them are almost like a blueprint in meaning. And so it has been for more than one year. People, who is there a site search bar for?
Secondly, the Internet is full of all sorts of nonsense, but a lot of good advice. I still do not understand how people by 2020 do not know what is the calculation of calorie intake, etc. It's so obvious!
So. In order not to bother with theory at all, there are ready-made calculators on the net. For example, this one. The accuracy and adequacy of any such calculations is relative, but we don't need more. If you buy packaged food in a store, then the calorie content is already written on the pack. Convenient, but you also eat unpackaged food, cook yourself, etc. There is a good site where you can find out the calorie content of products and ready-made meals, based on reliable sources, and not data taken from the ceiling, as it happens on the net.
Also, the calculation of the BJU. If you get the right amount of calories from food, then this is not a victory. You need to try to eat healthy, easily digestible foods, for example, rice, buckwheat, chicken, fish, etc. It is important to maintain a balance of protein-fat-carbohydrates. If your diet lacks carbohydrates and proteins, then muscle growth will not be optimal. If you have a lot of fats and carbohydrates, you will gain body weight from adipose tissue, which is not good. The simplest proportion is 30% protein - 30% fat - 40% carbohydrates. But this rate is floating, depending on age, gender and intensity of physical activity.
Of course, physical activity. If you do not burden your body, then the bulk of the protein will simply go past the checkout. Here is a healthy sleep and the correct training method.
Now, when you observe all this, and after conditional 2 months you will not have at least approximately the effect that you calculated theoretically, then this is already a question for endocrinologist.
I'm not an expert in this area and, in principle, far from medicine, but I will express my opinion.
Apparently you are an ectomorph. The most important thing here is the absence of a sharp weight loss and the fact that everything is fine with health. You probably always weigh that much, plus or minus. Given all this, the question arises, why do you gain weight?
There is an option to use chemistry, proteins (in your case, a gainer is better) and build a training program for gaining mass. But there is always the possibility that by stopping the use of chemistry, your weight will return to its previous norm. Taking this into account, there is a second option, more correct - not to use chemistry and just go in for sports. In this case, you most likely will not gain weight, but you will not harm your health.
In general, to gain weight, you just need to get more calories than you spend. On paper , everything looks simple, of course, but in reality it is not so easy for an ectomorph to do it.