Nadal is the undisputed GOAT of both clay and Roland Garros, with no remotedly possible argument for anyone else. He is the most clear cut case of anyone for a particular slam or surface, man or women.
Navratilova is probably the Wimbledon GOAT, but she is not the undisputed grass court GOAT. She averaged only 6 slams at the 2 grass slams vs 6.33 for Court at the 3 she played, 7 for Graf at the 1 she played, 7.5 for Wills at the 2 she played (she never played the Australian). French Open and clay GOAT for women is probably Evert, but some would say Graf and Seles at their best were better clay courters, and faced way tougher competition on clay, in Graf's case even with the Seles stabbing the clay field of her era >>>>>> the clay field of Evert's.
Obviously for the men the grass and Wimbledon GOAT is debated amongst Sampras, Laver, Tilden (grass only), Gonzales (grass only), and Federer.
Hard courts is probably Federer for the men, but it is a surface that has only been in high prominence for 35 years anyway. U.S Open GOAT is probably Tilden, but could be Connors, Sampras, or Federer as well.