Hold on Junior!! If the head gasket is leaking chances are bout 99.5% your coolant is puking itself out the overflow!! If that ain't happening then LEAVE the gasket in place.. Coolant don't get into the combustion chambers unless combustion pressure gets into the coolant and blows the coolant out the overflow.
What most folks don't know about torking down heads is the shape of certain cylinders changes every time the bolts are reinstalled.. That can be a bad thing.
So make SURE you have a leaky head gasket before touching it.
The smoke can be any number of things. Do you notice a loss of coolant which leads you to think of a head gasket? If so Look for an EXTERNAL leak and not an internal one..

But to fully answer your question.. Head gaskets are replaced with the engine in place under the hood.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />