Hello
You should determine which one is the physical interface by checking which interface (again, physical) will be used to router GRE packets towards the destination.
For instance, you state that your tunnel configuration is as follows:
interface Tunnel0
ip address 10.0.0.1 255.255.255.252
tunnel source FastEthernet0/0
tunnel destination 192.168.1.1
If the destination ip 192.168.1.1 is routed via your serial interface, then the physical interface that you will use to apply your Output service policy is SerialX/X.
Your setup seems correct. You only need to review if your policies are correctly configured for the pre-gre header or the GRE encapsulated packets (as stated in the documentation
Adolfo