Tiling on Drywall in Shower
Tile over dry wall even water resistant will not last as long but mat survive for up to 25 years if well sealed and has a gap at the bottom of the tub. 
This method of tiling was quite popular in the 50’s, 60’s and into the 70’s . On repair work I have done on tiled bath and shower stalls neglect is usually the cause of the failure. What was the neglect? Failure to reseal the grout so water finally degraded the Sheetrock to the point tiles fall off or someone pushes the wall in , the larger the tile the longer the wall will last. 
So can you do it , yes , we flipped several houses with a quick overlay that made a crappy looking bath look like a mid level home. 
On my own houses I have always stripped down to the studs, on showers I put up poly vapor barrier, not on tub surrounds, then cement board and then tile. My very first tile shower is over 45years old and the only problem has been the fixtures failed and they were tough to replace as I tiled right up to the valves. (My mom still has this home) last time I was home I did acid etch and reseal the entire thing as my step dad can’t do much any longer but asked if I could make sure it would be ok, Mom has always been worried as it was my first shower back in the 70’s. So if done right they really can last a lifetime and this was 2”x 2”.