This is certainly interesting and I can reproduce it.

I do find that if I query the GATEWAY role directly it shows up the MR2 configs. Given that MR2 is a purely GATEWAY + JOBHISTORYSERVER aspect in YARN, this would make sense, but I agree the service-level exhaustive config ought to show it too, and I'll report this internally to see what's up and if it can be changed to show the values in the future.

Something such as the below would work in the meantime:

curl -k -iv -X GET 'http://admin:admin@HOST:7180/api/v12/clusters/cluster/services/yarn/roles/YARN-GATEWAY-1/config?view=full'

You can grab your gateway rolename from the services/yarn/roles endpoint first, in case its something custom. In my example above the rolename was "YARN-GATEWAY-1".