custom queuing is not supported in GRE Tunnel interfaces. You can use a rate-limit command on them which might get you what you need, as far as QOS functions go.
7600(config)#int tun0
7600(config-if)#rate
7600(config-if)#rate-limit ?
input Rate limit on input
output Rate limit on output
7600(config-if)#rate-limit out ?
<8000-2000000000> Bits per second
access-group Match access list
dscp Match dscp value
qos-group Match qos-group ID
Hope this helps you,
Don