Upvote:4
First, be aware that a traveler would have to purposefully cause this or be really, really...um...let's say distracted.
You can't board an international flight without showing some travel docs. (For the nitpickers, I'm sure it's happened, but shouldn't ;) Meaning, one would have to show the Passport to the Agent, then put it away in a bag before leaving that same bag to be taken to the hold.
I'm most cases (I've seen it a lot), the gate checked bags are tagged before the traveler boards and the Agent asks a number of questions about medication, batteries, valuables and Passports.
In cases where bags are checked due to unavailable space, the Agent should still ask the questions and double check the traveler has all their documentation.
However, if the traveler manages to navigate past all these safeguards, their only option is to inform the Agent meeting the aircraft or other airport personnel to explain the situation. The bag will be retrieved or the traveler will be escorted some very annoyed personnel and/or officers.