TfL are managing their own train services. If GWR want to run it to the line for whatever reason they muight offer, that's their choice, - it's a privatised railway. Anybody looking at the TfL timetable who is used to the current level of service would (if they have any sense at all) investigate further. That is no big thing. The main aim of GWR is to encourage people to use GWR trains. Similarly, the main aim of TfL is to provide transport services to London passengers and visditors, and encourage them to use TfL services.
It might not be helpful to some but the system was set up with chinese walls in the name of competition and profit, - not my preference. I do remember though when it was LTE and BR, (not west of London) that stations served by both would have separate timetables posted on the same platform. The format and presentation was different but anybody using the trains soon learnt to read both.
This practice is not new and not really an issue for almost everybody.
If it's not your preference, then why on earth are you so determined to defend this selfish, compartmentalised bit of nonsense on the part of TfL?
GWR are also quite keen on doing all that they can to encourage use of the railway full stop, not just their own trains, whatever you may think their main aim is supposed to be.
The poor deluded fools have included all TfL services out of Paddington in timetable T10 ever since they started operating to Hayes and Heathrow, rather than taking the juvenile attitude TfL has adopted.
I'm enjoying this very quaint notion that passengers still actually pick up timetables to plan their journeys...
Ah yes, how quaint to be able to see a whole day's services on a route at a glance - or check a train time while in a mobile phone or wi-fi blackspot, or when your device has a flat battery, etc.