The difference between Deborah Tannen and John Gray is that Deborah Tannen is a well-known and respected communication researcher whose work is based on a 20-year career of sigificant research. By that time, she'd earned her spurs in the academy and could "afford" to write a popular work, which is looked down on.

John Gray is regarded as a popularizer without substantial research to support what he claims. He is very, very well-read.