I've found that hovering the heat gun back and forth along one end of the screen for about 30 seconds, two inches from the glass and on low heat, is enough to loosen it up. You'll notice the rubber getting sort of "glossy" as it begins to melt, so when you see that, do not apply more heat to it, or at least do not touch the rubber, because you will permanently affect it if you touch it.
I don't know if your glass screen cover is broken or not, but be aware that as you pull away the glass, you'll see a reflection of the glass in the screen itself, which is a pretty freaky experience, especially if the glass screen cover is broken into shards, because the reflection of the shards gives the constant impression that you've destroyed the screen.
Anyway, all I can say is, go slow, don't heat it too much, and in pulling on the glass make sure not to put pressure on the screen...it's pretty easy to break the screen doing all of this.
Not a fun procedure! I still occasionally find powered glass fragments on part of my kitchen floor from doing a few of these.