The Tile Council of America does not recommend the use of green board as a wet substrate for ceramic tile.
Gypsum based products promote mold growth, especially in wet areas.
Gypsum based products deteriorate in wet areas.
Tile and cementitious grouts will not prevent the substrate from getting wet.
Their use in semi-dry location, e.g. outside of a shower or tub is fine but if you are looking for the ease of installation a drywall panel offers why not install a product approved for use as a wet substrate instead. Example: Dens-Shield.