Just run into this interesting post concerning the monitoring of MGMTDB databases as part of Grid Infrastructure, on Neil Chandler’s DBA Blog
However as we know that Oracle 12.1.02 comes with a mandatory Grid Infrastructure Management Repository database (the MGMTDB database) it to my opinion makes sense to monitor it using Oracle Enterprise Manager.
For more information on the GIMR database see the Oracle documentation on it: http://docs.oracle.com/database/121/CWADD/troubleshoot.htm#CWADD92242