I'm not sure what you're asking. It sounds like you want to evaluate a bolted gasketed joint without modeling the bolts, the gasket, or half the joint. Gasket seating stress will be a function of the stiffness of both flanges, bolt stiffness, bolt spacing, etc.
I've never done this before, but it looks like in Workbench you can model the gasket as a 3-d body, and set the stiffness behavior to gasket. You can apply bolt loads with a bolt pretension object inserted with the rest of your boundary conditions.
Rick Fischer
Principal Engineer
Argonne National Laboratory