Couldn’t get it open with anything, including dunking it (wrapped in a ziplock) in freshly boiled water for from 2 to 20 minutes. But then I managed to get it with only minor discolorations along top and bottom edges (which are noticeable when the screen is off or on dark backgrounds).

What did the trick was setting the heat gun to 100°C and heating it really up close for 2 minutes. 60°C didn’t do squat, but 80-90°C would probably work better, without damaging the screen.

With that done, the glass required a little bit of force to lift up, force of gravity on the rest of the phone wasn’t enough.