I'll go with no GOAT of grass, but greatest grass courts players of their eras. Federer, Sampras, Borg, Laver would certainly be in that list.
I just don't like GOAT claims, and being a Federer fan it would be easy for me to say that Fed is GOAT, with so many numbers to his name. I just don't like comparing eras, things are always changing. I am content with players being the greatest of THEIR time.
Society is breeding a new kind of troll...But this is where the trolling stops...and I start.