There are a huge number of Oxford guys who go through GECAT, everyone on my assessment was Oxford, except me, and I got through. It doesn't really make much difference, but I think the majority of the CVs they put forward for assessment are from recommendation from OAT. I think they get inundated with them.
The IPS and GPP are one of the same thing. Most people start on the IPS and are either put forward for jobs during or shortly after the TR.
This is what I have been told.