You don't need to replace the gasket but why would you not do that? It is cheap and doesn't hurt anything. if the old gasket is still there than yes you can skip it as long as the old gasket is intact.
Regarding the strut nut torque specs I think it is pretty low like 22 ft pounds or something like that. I just tighten until not loose and this has worked on all the struts I have done. It is pretty hard to get a torque wrench on the nut while holding the center shaft.