Adding a couple data points from
this thread on Gravel Cyclist. wgscott reported using a GRX crankset with 6800 Di2 derailleurs, but had to nearly max out the high limit screw. Nebo used a GRX with 105 (presumably mechanical) equipment, and reported that the trim is now useless but it works otherwise. hbs reported using GRX with Ultegra equipment, works great, no details.