What is the best way to remove pvc from a shower mixing valve? I used it for the tub spout instead of galvanized steel. Should I use a heat gun in a tight location?

Channel locks should work. PVC doesn't gall into the mixer like some galvanized pipe can. What's wrong with the PVC spout? I've seen people use sprinkler riser pipes.

Tried channel locks but the plug is somewhat stripped. I had seen videos of people using a heat gun to melt it but didn't know if that was an option in a tight space

It probably won't hurt. Let's get this straight. I believe you used a PVC plug to fill the outlet of a mixing valve. Correct? If so, I would carefully drill a hole in it and use a pipe nipple extractor. You can find them at HD in the plumbing section and possibly in the Lawn Sprinkler section for removing pipe risers.