I may be able to help you out here, if you're not in a tearing hurry.
I believe the gasket you want is a three-layer metal one, GM part number 24443291.
I have a spare that I'm very unlikely to want; it came with a headgasket kit, but I didn't remove the exhaust manifold when doing the HG, 'cos it had been off a year or so earlier, and had fresh gasket, studs and nuts which I preferred not to disturb again.
Yours for the postage (should be cheap) plus a quid.
As for removal/refitting, not too bad. I was pleasantly surprised when I did mine, some of the studs just unscrewed with the nuts, all came out without trouble, except for the one that was already snapped

. That's another story.
Disconnect manifold from downpipe (new bolts + gasket recommended), shift heatshield and a few other bits out of the way, and you're in.
May be worth a close look at the thermostat area before you buy bits, new o-ring/sealant advised if there's any sign of leakage here.
PM me if you'd like the spare gasket.
