Is the cable pull of Shimano GRX FD-RX400 same as the one of 10- or 11-speed road groups?
11 speed GRX 810 and 600 are compatible with 9000, 8000, 7000, 6800, 5800 etc. 11 speed road groups - so it obviously uses the 11 speed cable pull ratio.

10 speed GRX 400 is compatible with the Tiagra 4700 10 speed road group only, so it uses the ‘11 speed’ 
cable pull ratio as well.

Shimano is nice enough to publish very comprehensive (if complicated) compatibility info: https://productinfo.shimano.com/#/com?cid=C-453&amp;acid=C-454.

Look for Drivetrain for ROAD, Gravel/Adventure, URBAN (METREA) components then Rear drivetrain compatibility and 
Front drivetrain compatibility.