I'm not familiar with any problems Hyundai heads have, but some things I know about head gaskets are:

The head must be flat, less than.004" warpage.
Overheating the motor just once will warp the head.
Clean the block with a wire wheel and sandpaper.
Make sure the radiator and thermostat ok.
You can only mill so much off, a machine shop can measure that.
50/50 antifreeze mix is a must.
Gasket sealer probably won't help.

Some things to try:

Have it checked for cracks.
New head bolts (some motors require that)
After a few days and heat cycles retorque the bolts.