Hi Gaurav,
The GOS for the transactions IW21,IW22,IW23 are linked to the object type BUS2038. If your workflow has an instance of BUS2038, then it will appear in the GOS workflow overview.
You could instantiate BUS2038 by using the BO: SYSTEM, method: GenericInstantiate and bind it to the workflow container of type BO BUS2038.
Or
If you are triggering the custom workflow using SAP_WAPI_CREATE_EVENT function module, then you could create an instance of BUS2038 in the BADI where you trigger the event & pass it to the INPUT_CONTAINER. Bind it from Event Container to Workflow Container in the start event tab.
Thanks,
Karthikeyan