If your putting a new back glass in it and a new gasket what does it matter?
If you want to save the gasket and glass here is what you do.
from the inside at the top, in one of the corners, start to move the corner of the gasket down and away from the pinch weld, while doing this push lightly on the corner of the glass you will see it move a little bit. While keeping light pressure on the glass start to move along the top pulling the gasket away from the pinch weld as before then slide your hand over and put pressure on the glass in that area. then move down the side doing the same thing. Or if you only need to save the glass, from the inside cut the rubber gasket from around the pinch weld and push the glass out. You might want to have someone hold the glass so it wont fall out and break