I'm not sure what gasket you're talking about, but about the grease, here's my opinion. If the car has a lot of miles, I would remove the pressure plate and inspect the clutch disc. Also, I would probably replace the throw-out bearing and pilot bearing if nothing else. These parts are relatively in-expensive considering that you already have the transmission out. If you decide not to replace any parts, then I would insert a small amount of new grease into the pilot bearing while being sure not to get any (least possible) on the clutch disc. This is the only place that I would consider if it was my car. Others may have different opinions, but I don't like the idea of grease in the bell housing. If it migrates to the clutch surface you will have problems.
Hope this helps
steve