HeatnBondLiquid Vinyl Water Proof and Stain Resistant Gel Coating, 8.68 oz

Q: Can you use a heat gun instead of the iron?

I do not know. In the instructions, it says to use an iron. A heat gun is not in the instructions as a suggestion but may still work. Good luck if you try it out

Q: Does this make fabric stiffer?

It will change the texture of the fabric a little. But it will not be too stiff to stand the fabric. I used lightweight interfacing to make a medium-sized cosmetic bag and it was just right.