Heat shrink wraps would be great for your situation. They can build up the grip size by 1/8 ( Black color ) and 1/16 ( White color ). Links are provided below.
1/8 build up :
1/16 build up :
For the heat gun, just use a hair dryer. Or a oven or stove. Tried the oven and it worked pretty well and fast. I just kept spinning the racquet in the oven since it covered 2 sides of the racquet. But just make sure not to burn it or melt it to a point where it starts dripping. <-- From my experience.