I think I'd start with a compression (or leak-down) test to determine if pressures are correct. IF not, it would indicate assembly error (gaskets or cam timing).
Can you scan for errors NOW at least? That should give you a mis-fire code pointing to a specific cylinder (hopefully), or you may get a "random" mis-fire code at worst.
Any "story" leading up to the head gasket replacement need may help us out. Specifically which engine in discussion here as well.
Plug in your location under "USER CP" too. We get requests from all different vehicle origins from around the world. Some differ in specs. And welcome aboard!