GAPS is gut and psychology syndrome. Yes, it's about leaky gut. It's a protocol to heal the gut (basically, simple healthy food). Theidea is that once the gut is healed it will stop absorbing undigested food which act as toxins in the body, it will be able to properly absorb nutrients so the body and brain can function properly and it will be able to remove toxins such as heavy metals from the body, the way a healthy gut does. The book is excellent and I highly recommend reading it.