Gravel bikes are here to stay, and anyone who says otherwise has Shimano to answer to. The Shimano GRX gravel bike groupset adapts typical Shimano road technologies to mixed-terrain use. One of the most important components of any road adoption to gravel is the inclusion of a wide gear range. The Shimano GRX RD-RX815 Di2 Rear Derailleur and RD-RX817 Di2 rear derailleur takes care of that and then some.
Simply put, the Shimano GRX RD-RX815 Di2 rear derailleur is a combination of the Shimano Ultegra RX rear derailleur and a Shimano mountain bike rear derailleur. It features a slim Shadow profile and clutch mechanism but has a slightly stronger clutch compared to Ultegra RX. It is also very slightly heavier than Ultegra RX, but the overall quality matches that of Shimano Ultegra. The Shimano GRX RD-RX812 rear derailleur is 1x-specific
The Shimano GRX RD-RX815 Di2 rear derailleur for electronic shifting is optimized for Shimano 2 x 11 drivetrains, though it can also be used with narrow-range 1×11 Shimano drivetrains as well. It is compatible with all Shimano Ultegra R8000 Di2 and Dura-Ace R9100 Di2 shifters.
For wide-range 1 x 11 Shimano gravel drivetrains, Shimano recommends the Shimano GRX RD-RX817 Di2 Rear Derailleur, which can easily run an 11-42t cassette option.