The trains will enter service in spring 2019, initially supporting the introduction of refreshed Bombardier Electrostar EMUs on Heathrow Express services, although the fleet will primarily be employed on Reading - Gatwick Airport and Reading - Oxford services.
GWR says details of how and where the trains will be used are currently being drawn up and these plans will be finalised later this year.
The class 769s are being converted from class 319 dual-voltage EMUs recently made redundant by the introduction of new Siemens class 700 trains on the cross-London Thameslink network.
The trains for GWR will be internally and externally refurbished with free passenger Wi-Fi and power sockets at every seat, and more luggage space. Each four-car set will be equipped with two diesel engines.
The delivery of the class 769s will enable GWR to redeploy class 165/166 Turbo DMUs currently used on Thames Valley services to the Bristol area.