In order to install SCOM 2007 agent on a workgroup computer there are several scenarios. the most common one is to use a gateway server which can be used as a management server in the DMZ environment.
but there are some scenarios in which we want to monitor only few servers, for example Exchange EDGE and ISA servers and we don’t want to deploy SCOM Gateway server, in this scenario we have the option to monitor a workgroup computer without using the gateway server of SCOM 2007.
In order to monitor the servers in the DMZ or any other workgroup environment, we need to perform the following steps which described in KB article 947691.
Link: When you try to install a System Center Operations Manager 2007 agent on a workgroup computer without using a gateway server, Operations Manager 2007 cannot see the workgroup computer