GWR is indeed having to offset the additional mileage between Redhill and Gatwick by savings elsewhere. You will see off-peak on Weekdays the Didcot to Oxford turbo shuttle is reduced to hourly.
I would argue that since you now have the fast trains from London to Oxford stopping at Didcot, running the shuttle on this side of the hour makes no sense, when most passengers will just take the faster, slightly earlier train anyway. The other side of the hour can pick up the calls at Culham, Radley and Appleford, which also results in a nice even hourly timetable for those stops (or 2-hourly for Appleford), so this is smart usage of resources imo.
The only odd thing that this causes, is that to make sure the turbos are used efficiently, the Didcot calls on the Oxford to London services are now in the services that come from Worcester, and the service that starts in Oxford calls at Reading and Paddington only - not ideal, but I still think this is for the better since it frees up a unit for elsewhere on the network, that was previously not being of much use.
I am also impressed at the Reading to Gatwick journey times - they look like they are roughly 1h30, when the "fast" service currently takes 1h25 anyway - I'm sure every single passenger will welcome 2tph even if it means a service that takes 5 mins longer.