With IPHost Network Monitor you can run simple snmp requests against a Cisco device in your network.
cGgsnServiceModeStatus
Cisco Ggsn Service Mode Status
1.3.6.1.4.1.9.9.240.1.4.9
This object represents the internal service mode which GGSN maintains. The service switching is done with object cGgsnServiceMode.
Back to CISCO-GGSN-MIB MIB page.
IPHost Network monitor allows you to monitor cGgsnServiceModeStatus on Cisco device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Cisco mobile switches right now.