I have a CGW bushing in the freezer ready for installation, and so far - it seems the OEM bushing is on there like a rock. Although a new act for me, I sat the slide in some WD40 for about 10 minutes and gave it 10 minutes with the heat gun as well. Felt this was plenty.

Dropped an 11mm socket in the back of the slide, rest an extension on it, and hit it with a sledge hammer pretty hard. Not a budge.

Tap out and take it to a smith? Yes, I removed the front sight pin haha.

I've used the freeze spray on stubborn sights before. If you don't have that you can turn a can of dust off upside down and you get the cold liquid spray.

10 minutes with the heat gun and it seems like the bushing would warm up with the slide defeating the purpose of heating. Try freeze spray just on the bushing right before you whack the socket.

Use a slide hammer? Or make one?

try KROIL instead of WD40

Keep hitting it.  Mine moved a little bit at a time as I kept tapping it.