I converted a friends Jamis Renegade gravel bike (Tiagra 4700 10sp shifters) to a 1x10 setup using a GRX RX-400 10sp derailleur, with a 38t x 11-42t setup and works perfectly. The derailleur is rated to that capacity (as per the post above) but the b-tension is at maximum.
On my personal Renegade I'm running 1x11, with a 40t and 11-46 cassette. That is however with a Shimano XT M8000 11sp GS (medium cage) MTB derailleur + Wolftooth Tanpan. Clears is with a a few mm's of B-tension to spare. Compared the cage length and it appears the MTB 11sp derailleurs are slightly longer in the cage vs. the GRX stuff (makes sense, needs to compensate more for chain grown on full-suspension bikes etc).