Unknown messages are usually new messages that may be in use on your network but have only recently been added to GTPv0v1 by the 3GPP. These messages may be considered by the 3GPP as reserved or for future use. You can configure FortiOS Carrier GTPv0v1 message filtering to allow or deny unknown messages.
If you choose to deny unknown messages, you can create a list of unknown message types to allow if there are messages on your network unknown to FortiOS Carrier that you would like to allow. See GTPv0/v1 message filtering.