As of August 2020 The Model-Driven telemetry lab ships with IOS-XR 6.5.3
This XR version has some issue with the gNMI server that prevents it from accepting YANG list keys without extra quotes around key names.
If one would want to get the interfaces information using the openconfig models they would do this like:
gnmic -a 10.10.20.35:57400 -u admin -p Cisco123 --insecure -e JSON_IETF get --path "openconfig-interfaces:interfaces/interface"