My heat gun has two settings, high and low, low is more than enough to do the job. You'll have to keep moving it around to keep the finish from burning. It seems to be fine while on the guitar, but when it starts to peal up it will smoke and burn if it gets too hot.
I tried a putty knife, it does work, but takes too much heat to get it to move freely. A regular non-serrated steak or pocket knife worked the best for me. Tried a razor blade, but it digs into the wood too easily I would not recommend using one.
Once the finish gets warmed it really does peal off easily.
I will admit, this is the first project I have done of this sort, others may have better techniques for removing the finish, but it worked well for me. Go easy with the heat.
Oh, a good pair of gloves or oven mit will come in handy.
Does your heat gun have settings for temperature (the one I have does was wondering if you knew what a good one would be) Any do you think I should use a grinder to dull a putty knife or just use a butter knife, by the way thanks for the pics it looks like that is the same guitar, would love if you would send me other pics of your progress