Greenboard has lost its ANSI certification for use as a wet substrate for ceramic tile. The 2001 Tile Council of America also has dropped it from their wet substrate list. You would be better off using 1/2" Durock or HardiBacker. Install to walls studs over 15 pound roofing felt (vapor barrior) with screws. It may cost more now but cheaper than tearing out and replacing 2 years from now.