Members of the GNR are military personnel, subject to military law and organisation, unlike the agents of the civilian Public Security Police (PSP).
The GNR is responsible for the preventive police and highway patrol in the countryside and small towns of Mainland Portugal (large urban centers and all the Portuguese islands territory being patrolled by the PSP). At national level, GNR also has duties of customs enforcement, costal control, nature protection, search and rescue operations and state ceremonial guards of honor.
Since the 2000s, the GNR has provided detachments for participation in international operations in Iraq, East Timor and other theatres.
Source: Wikipedia