I'm not a metalhead, so didn't know Cemetery Gates is a song and therefore my initial interpretation was that they're playing a solo on a cemetery gate, as an instrument.

I guess, open and close it for that sweet squeeking sound and maybe use it for percussion, as the world's biggest triangle.

And the thing is, I wasn't entirely sure, that's not actually a thing in metal. 🙃

If you ask me he's playing something more chill and introspective. Maybe something like this?