Well I don't know about a heat gun, but I have always found it quite easy to do using an Unger 4inch fold away razor scraper.
You have to be careful, very easy to snap the blades, providing the stickers are not to old and have perished and become brittle, if you are careful you can start an 'edge' and slowly peel off, using the blade against the glass to keep it going.
If there is adhesive left on the glass, spray or wet with your normal window cleaning solution and use the Unger scraper, with the 4inch one you can fold the blade back into its cover and clip into most standard squeegee handles, so you can also use it on your extention pole.
There are 6inch scrapers, but they are not as good as the 4inch ones.
I haven't used a heat gun, but I think that may well take longer, particularly if it makes the adhesive soft and sticky again
Ian