The 769 is an updated 319 with some diesel packs bolted underneath a few carriages to make it into a DEMU▸ (I think that's what it would be called!)
As such they are ideal for GWR▸ , as they can be used under the wires, on 3rd rail (like various sections between Reading and Gatwick) and away from both - possibly to Oxford, Basingtoke and maybe Bedwyn?
Almost, I think they'd be more appropriately classed as an EDMU as they are primarily electric, but that's splitting hairs.
Anyway,
AFAIK▸ the first round of the 769s will be just 25kV and diesel, with the ability to run on 750v
DC▸ to follow. The useful thing about the 319's is that the 25kV is transformed down to 750v, with a power bus running down the train. So the concept of the 769's is that the diesel engine and generator sets effectively pump 750v into that power bus, as though the unit was running on the third rail, so you can more or less use the same control gear, with a bit of extra kit in the cab to keep an eye on the engine. As ever though, the devil is in the detail and I believe that you need some fairly clever electronics to keep the two gensets from upsetting each other, which amongst other things is why they're still not in service yet.
The 3rd-rail compatible 769's I believe need some extra switchgear to keep the engines isolated from the shoegear, no idea how far that has progressed yet.
As for usage, remember that in the modern railway everything has to be cleared to the nth degree, drivers have to be trained etc, so I would expect a relatively narrow swathe of London to Reading and then Reading to Gatwick once the 769 conversion is in place.
The diesel engines are fitted on a raft under the trailer