Hollowgram crank + GRX

Main difference is the GRX front derailleur has a 3mm offset outwards and expects a 46mm chain line. If your current axle length supports it you could add 3mm spacers on at least the drive side. That will however offset your Q factor on that side with 3mm as well. To compensate for this you could buy the special XTR pedals that have 3mm shorter axles normally used in combination with MTB boost drive trains.

If you want to keep your existing Q factor, pedals and hollowgram cranks you can also opt to go for an Ultegra Di2 8050 front derailleur, according to https://bike.shimano.com/en-EU/product/component/ultegra-r8050-di2/FD-R8050.html that supports 16T difference and 46T for the big ring just like the GRX cranksets.

You can mix and match all Di2 GRX, Ultegra 8xxx and Dura Ace 91xx components, they all support E-tube & synchro shifting.

Give thanks!

that answer helped me a lot!

regards

I've run both the Ultegra Di2 FD suggested above with sub-compact gearing (both 46/30 and 48/32) and the GRX mechanical FD with a non-Shimano crankset having a standard chainline and 46/30 chainrings. Both function well. However, I do feel that the GRX FD works better for the sub-compact gearing (46/30 and 48/32T), even when paired to a non-GRX crankset.