I can't speak for GP, but when it's combined with talk therapy, the ketamine allows the body to lower its defenses so that talk therapy can more successfully be administered, and the combination of the two is what leads to long lasting effects, long after the ketamine has worn off. Ketamine let's the patient open up so talk therapy can get at the underlying issue and resolve that, in ways that talk therapy alone can't solve because the patient's flight /fight response/anxiety won't let them let their guard down to really talk about it.