The last time I had to use any sort of sealer on a head gasket was on a 1943 IHC GI pickup truck - the "gasket" was a stamped steel shim that we reused (nothing wrong with it, after all.) The sealant was spray copper.
Modern head gaskets are usually composition - either single-layer or a "sandwich" setup - with sealer "printed" on them for the water passages and "fire rings" to seal the bores against compression/combustion. Adding sealer to that can actually interfere with head gasket sealing in the first place, so don't bother. I've heard mixed results if a sealer is added, but they tended more toward bad results than good.
If you have trouble holding everything in place, get a stick of 1/2"-13 threaded rod, cut off two bits about 4" long, and slot the ends. Thread into opposite corners, lay on the head gasket, lay on the head. Start some head screws, use a flat screwdriver to unscrew the threaded rod, and a magnet-on-a-stick small enough to go down the hole will pull them out for you. Throw the threaded rod in your toolbox for the next time you have to do the job on something else (I've got plenty of pairs of threaded rod - for just this very reason! Assorted sizes, inch and metric.)
