Cylinder Head Gasket

If I take the gasket off, should I replace the head gasket, or reuse the one I’ve got?

Replace it! You never ever reuse a cylinder head gasket on anything. The whole purpose of that gasket is to create a seal, which can’t be accomplished unless you can create a fresh impression. Depending on the vehicle you don’t reuse the cylinder head bolts either if they are stretch types.

Why are you taking the head and gasket off? If it were me, I’d replace it while I’m in there, but the experts here may disagree.

Update: What Dave G. said.

You NEVER EVER reuse a head gasket. Always replace it.

Replace it. Also, follow the procedure to torque the head bolts exactly.

What Dave said is true of ALL gaskets. The difference with the headgasket is that because of it’s extreme service environment it WILL fail if reused, guaranteed, and its failure just may cost you an engine.

Other than the advice you’ve been given I will only add this.
Make sure the cylinder head surface is flat and resurface it if needed. You did not state what type of car here but normally if the warpage is over .002 per linear foot of the cylinder head it should be resurfaced.
If the head(s) is aluminum I would recommend spraying both sides of the new head gasket with aerosol Copper Coat. The epoxy greatly helps in sealing and the suspended copper particles help to fill in any minute nicks and pits along with aiding in heat transfer.

And of course, make sure the head bolts (replace them if torque to yield bolts) are properly torqued in stages.