If the faces of the surfaces are in good condition and true, then no sealant is required. The gasket
will be sufficient.
Don't worry too much about an exhaust leak with a diesel engine
. First there is only a small amount of CO in a diesel
exhaust when compared to a petrol (gas) engine
. Secondly the smell of the smallest leak of in a diesel exhaust will alert you a long time before it becomes dangerous.
Put another way, the smell of a small leak in a diesel exhaust leak drive you out the cabin