I used to manage a GNC store many years ago. I found that a good protein supplement was preferred by most over weight gain products. At that time the best one we had was of three sources but that might be different now.
As a nutritional consultant I always recommended that to gain weight one must eat healthy, not fats and sugars as people suppose. Lots of the same things you eat to stay healthy but more of it.
If you have kidney problems or some other health problems a high protein diet might not be for you. As always your doctor should be consulted before starting the diet. Most people also took a good all around vitamin too. Watch the GNC staff as they will sell you somethingmore than you need. When you have more protein you need more minerals.