I pronounce it GRAY-sin, and I have never used the -a- spelling in my life; it's always grey where I live.
Gray looks very foreign.
I would never use
Grey as a given name; it seems ludicrous, much sillier than all the
Indigo, Scarlet(t), Blue etc names. I used to know a
Greyson who is in his early 20s now; he was never called by any nickname.