First step is to enable SNMP on the gsa with a well choosen community string.
You also need the two MIB files from google, the GOOGLE-MIB and the GSA_MIB.
Copy the MIB files to your OpenNMS machine into the directory $OPENNMS_DIR/contrib/mibparser/mibs.
Next step is to convert these mibs into OpenNMS configuration. With the mibparser we can do this: