Hi Tom,
If you haven't verified the pg_hba.conf file for this connection, I would suggest take a look at below document:
https://gpdb.docs.pivotal.io/590/admin_guide/client_auth.html
If you are sure the pg_hba.conf file is correctly setup, then provide the complete error message may help check further. (You can find the complete error message) from master log. $MASTER_DATA_DIRECTORY/pg_log/