No Goat Links, no Tanpans, no 3rd party mods. An allShimano solution to an affordable, reliable, 11 speed GRX mullet setup for up to 1051t cassettes.

The GRX RX822 SGS 12 speed rear derailleur is compatible with GRX 11 speed shifters like GRX 600 and GRX 800. You install the RX 822 like any other derailleur, but max out the high limit screw, which limits it to 11 speeds. You will likely need a longer chain to go with your wide range cassette of choice. This video features an 11 speed Deore 1151T MTB cassette.

Shimano GRX RDRX822 SGS 12 Speed Rear Derailleur:
https://shorturl.at/gsMV5 (Jenson link)

Shimano CS M5100 1151t 11 Speed Cassette:
https://amzn.to/47swQiT (affiliate link, thanks!)

Check out Maverick Cycles if you're nearby in Whittier, CA
https://maverickbicycles.com/

Credit to drinksta who posted this in the r/gravelcycling subreddit a couple months ago:
  / it_works_11_speed_1151t_cassette_12_speed_grx