Ah...
It was congestion mitigation as to why the train terminated at Redhill. NR were, rightly, trying to thin out the service a little at Gatwick to allow things to recover. When the decision was made (around the time the train was at North Camp), the fast lines through Gatwick were closed, and there was a train at every signal from Gatwick back to Redhill on the slows.
It was complicated by the fact that on the way back from Gatwick it attaches to the next stopper from Reading. I thought Three Bridges were going to shunt it over to Loco Sidings and back in on top of the stopper (which is due to run into London end of 0), but instead they decided to hold you outside until 0 was free and instructed the driver to go to the far end, causing an annoying reverse attachment that was then a pig to sort back out when it got back to Reading, as well as missed connections etc etc - 20/20 hindsight but if they’d said they were going to mess around like that I’dve fought harder for the train to go through...
Generally though the new TT works. It’s still a pain to recover from major disruption, but that’s more down to how it’s crewed, with the vast majority of trains out-and-back crews from Reading. There’s more to manage, but that’s not a bad thing in my book.