Do yourself a huge favor. Carefully remove all the tile from all
walls. Be careful not to break any. Lay down a drop cloth and maybe
some cardboard to protect any that you may drop. Once you pull one or
two off the wall, seven or eight will follow and you'll be wishing
you had 4 hands. Maybe get an assistant to help you.

After all the tile is off, pull off all that drywall. Drywall in wet
locations like showers or tub surrounds is a big No-No. Tiling 101.
Replace anything that was behind the drywall that has been comprised by
moisture (i.e. insulation). Let that area dry completely before moving
on.

As mentioned in the quality post above, install cement board. This is
the proper tile substrate for what you need. Do not use green board!!!

Reinstall tile, using a quality thinnest. Grout and finally seal your
tiles.

If you need to use the shower during this project (form pulling the
drywall to regrouting the newly reinstalled tile), install heavy
plastic around the shower to prevent water from penetrating your work.
Only remove the plastic once the grout as cured.

If done right, you will do this once.

Don't try and band aid this problem; you're only going to have huge
headaches.

If you don't know how to install cement board, or do tiling do some
research. There are plenty of great resources at your fingertips.
There are way to many scenarios to speculate about here.

Good Luck.