I know it’s not supposed to work, but plenty of folks have had success running bigger cogs than stated. For example, I had 11-40 on my GRX Di2 2x bike and it shifted beautifully. I’ve also heard GRX 1x can work with a 46T.
I’m thinking about trying a Garbaruk cassette with a GRX 1x set up…likely a 10-46 to squeeze out a bit more range. Has anyone done something similar?
All the Shimano 12s cassettes are 10-XX and considering the chains are the same across I’d say it should work.
I don’t run road or gravel though.
To partially answer your question I’m currently running SRAM XD 10-42 cassette with Shimano GRX 817 di2 rear derailleur in an 1x setup. Works and shifts well on and off that 10 cog sprocket.
Awesome thanks!! That’s my goal if I can’t find the parts to build up an Eagle mullet 
Yep, I am running a 10-42 sram cassette on an xd driver with a GRX long cage. No issues on the 10t.