what is gene's purpose in returning to Devon?

Did you mean after fifteen years? I think Gene was looking for closure that he never had. He returns in to find a sense of meaning to the summer of 1942 which culminated in his friends death,

"Looking back now across fifteen years, I could see with great clarity the fear I had lived in, which must mean that in the interval I had succeeded in a very important undertaking: I must have made my escape from it."

This hence acts as a spring board into the memories and flashbacks into the events that shaped Gene into the man that he had become.