You can apply tile to regular sheetrock without any worries provided the sheetrock is not subjected to water as the greenboard around the tub. I once moved into an old home with painted sheetrock around a tub and as (what I thought would be a temporary situation) tiled around the tub on the existing sheetrock. As other projects took priority, it was 10+ years before I tore out the tile and sheetrock and remodeled the bathroom.
I simply used the same mastic everywhere to apply the tile.
I simply used the same mastic everywhere to apply the tile.