Iso preperation

Yes, it must be considered for two primary reasons.
(1). A Gasket is a piece of Material that must be shown on the Isometric, identified on the individual Isometric BOM (Bill of Material) and counted for the MTO (Material Take-off) and later purchased.

(2). It is a physical object which occupies space (has dimension) therefore must be considered in the individual valve installation and multiple valve manifold space requirements.

I would think the Gasket type, material and thickness would be a consideration in the stress analysis of Flange tightness and the prevention of gasket leakage. However the Gasket would not have any impact on other Stress analysis issues.