The VMONGS are ground sense lines. Each VMON pair on the POWR1220AT8 and POWR1220AT8-02 consists of a differential pair for the Voltage Monitor pins. The high side or VMONx, is tied directly to the rail to be monitored, if less than 5V. The low side, VMONxGS is tied to the ground side "at the load". It is important to run the differential pair all the way to the load when using the trim features. When VMONxGS can't be run all the way to the load, make sure it is tied to GND.
Click on the link below to go to application note AN6074, "Interfacing the Trim Output of Power Manager II Devices to DC-DC Converters":
http://www.latticesemi.com/view_document?document_id=27198