Originally Posted by
Badger6
Your RD will be beyond its spec'd capacity with that 11-42 cassette and any tooth spread on the chainring that exceeds 10T. Going with a larger small chainring will help offset that, any little bit will help. I don't recommend exceeding the RD capacity though. Good luck.
I found this
lengthy thread on another gravel bike forum using a big cassette with the GRX 400. From what I'm gathering, 11-36t will be easiest to swap out with little to no issue and after that more care will be needed going 11-40/42t, but it is doable.
Honestly I don't need that much extra help in low gear if I'm riding a trail fresh, usually I can go uphill no problem with the 34t rear, but if I'm doing a lengthy road ride before hitting an uphill trailhead, fatigue starts kicking in and 34t starts to feel heavy on my legs.