Is the cable pull of Shimano GRX FD-RX400 same as the one of 10- or 11-speed road groups? What is the required cable pull for FD-RX400?

AFAIK Shimano road 11-speed front deraileurs require the same (larger) cable pull, while 10- and fewer-speed FDs expect lower one (with exception of Tiagra 4700, which uses the "11-speed" pull).

Does 10-speed GRX belong to either camp?
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.