Hello,
You will need to use gr-x/y/z interface to configure GRE tunnels. 'gre' is an internally generated interface to handle gre traffic & is not configurable. Its use is elsewhere.
On SRX devices, Service Processing is handled by SPCs while on M & T devices it requires services PICs therefore on SRX the SPC should be able to allow you to configure gr interfaces without a need to buy Services PICs.
Regards,
Rushi