A gateway must use fidonet.org for Message-IDs originating from the standard FIDONET zones 1-6.
However, for non-FIDONET zones, a gateway may use a more appropiate Internet domain. Of course, it is essential that multiple gateways for one FTN zone/domain use the same Internet domain for gated Message-IDs!!!
Of course, there may be different Internet domains for the sender's address (From header), but for Message-IDs a unique Internet domain for each gated FTN zone/domain is mandatory.