I have been trying to get System Center Operations Manager 2007 R2 to discover an IBM BladeCenter chasis through a gateway and I finally figured it out.
First On the device you want to monitor, set your SNMP public community string to point to the IP address of the SCOM Gateway server.
In the SCOM 2007 Administration console, choose Network Devices in the Discovery Managment Wizard choose network device and click next.
In the next screen enter the IP address of the network device you want to monitor and under the mangment server drop down choose the gateway server who’s IP you entered in the SNMP string earlier. The device should now be discovered.