Is boat in the water? Not that it SHOULD make that much difference.
Check the fitting from inside the boat. See if it shows any crack or other distress.

My advice: DON'T be using a heat gun on this.

If you're fixated on it, you could pour a bit of potable antifreeze into the drain, then seal it with duct tape. If you cant' do that because you (rightly) would worry about make the deck a skating rink, then don't worry about it.

Then I'd wait till Spring. I think you'll be fine.

If you fixated on it. Then gently, with something that wouldn't burn or cause any discomfort, heat the area from the inside of the boat. That will temporarily solve the problem.

What do you think your bildge pump does if it has water it can't pump-out because it's frozen -- it burns out, if you have it on. Solution, a little potable antifreeze in the bilge AND turn it off if the boat is out of the water.