I just did this the other day. After many methods of trying like you did, I read that getting it in a table vice was the best method of taking it apart, so I went ahead and bought this one off of Amazon. It ended up doing the trick quite nicely. That sealant/adhesive is some tough shiz.
I got it with this gauge as well as it's cheaper than the one you posted.
I still haven't done the OPV mod as the gauge is still on the way in, and I ended up needing to replace my boiler thermostat as it got damaged recently.
Best of luck to you! It will/does eventually come off. Depending on your strength, you might want to get assistance from a strong friend as it was still difficult even with the vice.
Had that lying around and don’t remember where it’s from but this looks to be the same
https://www.amazon.com/IRWIN-Tools-3-Inch-Clamp-226303/dp/B0001LQY44 I found it. I've used mine every day, even before I made coils or started using it for coils.
Mine is on a workbench in the garage but it could also be clamped to an overhanging kitchen counter top.
Irwin Tools Clamp-On Vise, 3", 226303 https://www.amazon.com/dp/B0001LQY44/ref=cm_sw_r_cp_apa_0Rk6BbQFA72CC
I grabbed this for 25 bucks. I just clamp it to my table with a piece of wood (to hopefully avoid marks) and it works fine for what I need.
You can get a heavier duty one with a base for not too much, but I like the portability. I can carry this in my backpack if I want to.