Is that drywall in the tub area? Is the tub also used for a shower? If so, I'd replace it with cbu (cement board). Is the entire area going to be tiled, or just within the tub area? CBU is supposed to have the seams taped with the special alkalai resistant mesh tape - not the same as used in drywall. That can be added during tiling - taping a cbu joint isn't the same as drywall joint - it is entirely for strength, not for show at all since it is going to be covered with the tile.
A joint bridged by the tile should have the special tape, then you can tile over the joint onto the drywall. If there is paint where you want to tile, if the paint is well attached, scuff it up to take off any loose or gloss, wipe clean with a damp sponge, then add the thinset and tile.
The existing drywall may have been 5/8" fire resistant stuff, and you installed 1/2". Might be breaking codes here, depending on the building. If it's an accumulation of paint and maybe wallpaper buildup, it will be hard to match, but you can try.