We are tiling our shower and we've found that our walls are not square. At the corner where the tiled wall of the shower meets the regular dry-walled wall, there is a gap that gets wider as you get further down the wall. Is there something we can do that will cover this corner and the gap? It will need to be able to adhere to both drywall, and polished marble tile? We're looking for any suggestions

I could definitely tell you something, if I saw a pic of the situation, post it if possible

What is the minimum and maximum gap and over what distance?

I am not sure I have the correct picture in my mind but the drywall should have met the backer board without any gap. The correct way to fix this would be to hang a new piece of drywall flush with the backer board. Usually install a piece of bullnose tile on the outside edge. If you don't want to tear down drywall a piece of bullnose may be your fix, normally 4" wide.