Happy Holidays!
I'm not sure to understand what you mean. I replaced an Ultegra crankset with a GRX 600, I didn't add any spacer and I don't see any play.Beside, road cranks and "Gravel" ones are made for the same BB size.
On one bike I have a GRX 810 FD with an Ultegra crankset and no issues. The GRX FD has a wider range and can be easily adjusted. I have an Ultegra FD with a GRX 600 crankset on anorher bike and it works more than reasonably well.The only issue is that the chain slightly touches the FD while on the smallest sprockets and I couldn't get rid of it yet.
On AliEx (Magene shop) you can read:Not Support T47-86.5mm and BB386 bottom bracket !