Hello Shaofei,
a C12816 is a very powerful router but it hasn't SPAN capabilities like a LAN switch.
You need to add a LAN switch with SPAN capability: you configure two ports in the same L2 vlan and then you can use SPAN to mirror traffic coming from the GSR interface.
A packet capture feature has been introduced in 12.4T for ISR routers but it is not available for GSR.
Hope to help
Giuseppe