The GRX groupsets seem to be compatible with common road groupsets (Tiagra, 105, Ultegra) which was also pointed out in this answer. Furthermore, a brief news article by Shimano states that the brake and drivetrain components between GRX and road groupsets are interchangeable/interoperable:
Fully Compatible Parts
As long as you maintain your current drivetrain speeds (10-speed versus 11-speed), the following GRX components are fully compatible and interchangeable with your existing parts: Shift levers, chains, brake calipers, bottom brackets, cables and housing, and Di2 batteries and accessories.
However, some additional brake levers BL-RX812 for the upright position were introduced with the GRX series about which I cannot find any explicit compatibility information.
As Shimano's STIs combine brake and shift levers, I am not sure whether
refers to STIs (including the brake levers which I am concerned about).
Are the brake hoses of 105 STIs compatible with the GRX ones? Could I extend the road groupset brakes (105 or Ultegra) with these sub brake levers resulting in a setup consisting of 105 drivetrain, 105 STIs and GRX sub brake levers?