I have googled for this info, but all I can find are articles preaching that
this is completely unaceptable..

Since 1) I have had bathrooms in 5 houses over the last 35 years with
drywall backer and never had a problem..

2) This bathroom is only for a guest area in the basement, and rarely
used...

3) I have already installed the greenboard
4) If this was a main bathroom, now, I would use cement backer board

I need the info as to how best to cut the drywall, and tile to butt up to
the bathtub.

Right now I have overlapped the tub flange about 1/2" with the greenboard,
which leaves about 1/2" gap to the tub surface.
I was thinking of running a bead of silicon to seal the edge of the
greenboard, then tiling over to about 1/8" of the tub,
then calking the gap between the tile and the tub.

Does this sound OK ?
Thanks for any info or experiences

Ray