Unscrew the gas plug and clean out debris. Add some gun solvent or CLP and push the puck to the rear. Rack the charging handle and let fly. Repeat.
If it hasn't loosened enough field strip the gun and use a long wooden dowel to push/tap it out. Be gentle.
I have to admit, I have been remiss about cleaning this gun. It was caked with garbage. The plug came out no problem, but that damn puck was stuck in there tight. I tried letting it sit in solvent, but that didn't help either. It's obviously air tight as the solvent level didn't drop while it was pooled up on top of it.
Anyway