Hello. Thank you for coming to JustAnswer with your question, and welcome. I'm sorry it has taken so long for you to get a response to your question.
You have a couple of questions in your post, so I try to address each one.
As far as the weight gainer supplement: Keep in mind that these are supplements for people who are body building. It is nothing but extra calories, protein and vitamins and minerals. You must be weight training for you to build muscle mass, whether you take you use the weight gainer or not. And if you take the weight gainer without exercising, you will only build fat. At your current weight your BMI is well within the "normal" range.
So, briefly to answer your question about building muscle mass:
You build muscle mass by using the muscles. The more you use them, the stronger they become. To build *larger* muscles, rather than just stronger more toned muscle, you would need to enlist the help of a Trainer who knows what types of exercises each set of muscles need; how often to exercise; how much to exercise, etc. When you are working out to build muscle mass, then it would be appropriate to take a weight gainer supplement. But just increasing the protein and calories from complex carbohydrates in your diet will work just as well, if not better. Ask the Trainer.
As far as bone density: From your description is sounds like you had quite a bit of damage done to your bones before you had the parathyroidectomy. Most people, after the surgery will rebuild bone dramatically (per studies done), however some people will not recover the all of the bone density lost.
Here is a link that explains: Osteoporosis and Parathyroid Disease
Also, I would advise you to seek the advice of your doctor before starting a strenuous weight training program. You may want to recheck the density of your bones before you proceed, just to be on the safe side.