Gaining

If you want to gain soft kind of fat, subcutaneous fat, heavy cream is a good idea, as well as other dairy products. From what I know, you should base your gain on rather healthy (but fattening) products, like avocados, nuts, peanut butter. This kind of fat is also safier to have, comparing to visceral fat that wraps around your iternal organs. However, you mentioned belly fat and unfortunately, visceral fat is the one that is accumulating mainly in belly area. I hope it can help you a bit

Don't expect to have all this happen quickly.
Getting fat takes way more time than you think it does.
Albee makes good points.
Lots had to do with how your body stores fat and your genetics.
For years as a gainer all my fat was very firm.
I lost close to 100 lbs at one point and when I decided to start gaining again a year later everything I gained was more flabby.
I started to get a double belly and it started to hang which had never happened before.
I was using heavy cream and ice cream before going to bed and I am sure that helped or had something to do with it.

Glad I could be of some help.