This can not be executed/processed from portal. The GRC workflow engine is different from normal R/3 workflow.
This requirement seems to be normal. However, it is not supported in portal.
What I did is, GRC is simply integrated with Portal. However, execution is made possible by calling GRC Work Inbox page. As soon as approver clicks on any GRC work item in portal, it opens one page which is a custom application created in GRC by name "ZLAUNCH" (this application is looked for as soon as you try to process any GRC work item from portal)
This shows simply a link on it and caption is "If you want to approve, click this link". Once the approver clicks on this link, this will open GRC work Inbox of the approver which displays all pending GRC work items.
So this way, indirectly GRC work items are processed by approver. But directly, it is not supported in Portal.
Hope this helps others :-)
Regards,
Faisal