John Norman also wrote a very interesting book that while similar to the Gor books in philosophy is not about Gor at all.

Time Slave - 1975

In this book, Norman presents his personal theories of human evolution, exemplified by the case of a modern woman sent back in time twenty thousand years or more; he mourns the loss of human evolutionary fitness and distortion of "natural" social relations which in his view occurred when farming spread, and farmers squeezed hunter/gatherers to the ecological margins.