How far do I have to lower pool water level to replace skimmer gasket?
Can I caulk the gasket when I install new gasket?

It is best to lower the water at least 2" below the skimmer. When you replace the gasket use a rubber silicon type sealant, It will remain flexible as the pool expands and contracts with temperature even if it freezes during the winter. Place the sealant between all materials, the gasket ,the pool liner and the pool walI. If you have city water , no problem to refill the pool. I have a well and did not want to waste the water (I had to buy the water and have it delivered by tanker truck ) so I cut one side away from a large totes tub, placed an old bicycle tube around the other three edges , pressed it against the sidewall of the pool and replaced the gasket. It was a bit extreme but it worked very well. The most important thing to remember is to take your time and do a good job of sealing the gasket. A leak between the pool wall and the liner will run down the side wall and may slowly wash away the sand below in the cove of the pool.Good Luck