Sorry for a late response.
I guess the call forwarding at the GMSC would be under Call Forwarding Unconditional (the others as I understand .. please correct if i am wrong .. would be at the VMSC??).
In which case in forwarding to the FTN (which is a MS) does it not need to know subscription info for that number?? And if it does where is this info obtained from (If the FTN was roaming in the GMSC area it would have this info in it’s VLR)