I don't think a heat gun is prudent. The resin can be degraded if it gets too hot. Boiling water is probably safe. Resin that most people use for props softens at 150 degrees or so.
I first balance the prop. Then I check each blade to see if its pitch distribution is what I want. If not, I then take a teakettle and pour boiling water on the blade and let it cool without applying any forces to it. That gets it to an equilibrium so you know that there aren't stresses within the blade fighting each other. I then put some reference marks on the prop. I use the odd numbers on the Prather scale, but a less lazy person would probably check the pitch at each Prather station. I use a gold- or silver-ink pen so the marks will show up on the black prop. I start with station 3, pouring boiling water from the kettle inboard of the reference mark while twisting the prop (just before twisting the prop, actually), and letting it cool. I let it cool under the faucet, out of which cold water is coming, although I've been advised to have patience and let it cool in the ambient air. Either way, you need to hold the torque on the prop until it cools. Then I work my way out on the blade, each time pouring the boiling water just inboard of the station I'm adjusting. As you go out, it takes less time to heat and cool the prop and less torque to bend it. Whenever I overcorrect a section or get otherwise messed up, I repeat the destressing for that section and everything outboard of it. This process takes awhile. I've heard of sticking the whole blade in hot water, but I don't see how you can get all the stations to the pitch you want that way.
I should mention that the "pitch" of a prop-- as if the bottom of the prop is a screw going through cheese-- doesn't have particular aerodynamic significance. It's just something convenient to measure. There is nothing magic about having the same number all the way out. Once you get a prop you like (by borrowing a good one, for example), you can measure its pitch distribution and reproduce it on another prop with your pitch gauge and teakettle.