Is there any information, or informed conjecture, about what services GWR are planning to run, when? Is it only Reading<>Gatwick? Or (some?) Oxford<>Reading<>Gatwick?
The reports I have seen, suggest; -
Reading to Gatwick & presumably on the same tracks the
Reading - Redhill stopping service.
Paddington to Oxford through trains i.e over and above the 387s running Paddington - Didcot under the "juice', although given that IETs are on order for this route, I suspect they may be just a few peak hour through trips.
Reading - Basingstoke.
Not specifically noted but assume that
Slough - Windsor &
Twyford - Henley branches would be 769 operated
The West Ealing - Greenford & the Maidenhead - Bourne End - Marlow branches won't take 4 car trains.
They could work Didcot/Reading - Banbury.
387s run to Swindon sidings now and are due to provide 'Rugby Specials' through to Cardiff in future, main line beyond Didcot won't be cleared for 769s but presumably could be.
If there are 19 units due and allowing for engineering spares, that suggests a max of 17 diagrames, would roughly work out at 7 for Reading - Gatwick, one apiece for Windsor & Henley branches, 2 for the Basingstoke line and assuming either 3 - 8 car or 2 - 12 car for Paddington - Oxford, makes a total of 17? (will probably be corrected very soon!!!)