aokanlawon is correct (+5), to quickly see which route patterns use this GW directly (not a good practice) you can use dependency records (upper right corner drop box) assuming you have dependency records enabled via enterprise parameters.
Can you send a picture of your "device----->gateway" page on cucm.
Have you ran a depency record search as chris suggested on the gateway? I suggest you do that so you see what the gateway is assinged to
If you have confugred a gateway and it has not been assinged directly to a route pattern, then you should see it when you want to add it to a route group.
thank you so much all , i found the issue , the sub unit wasn't in a device pool , it eas in none device pool ,and when i change the device pool coinfiguration for this port , the decice appeared in the list