Loyalty: In addition to Male and Female, Alaska includes “Unspecified (X)” and “Undisclosed (U)” as gender options on Mileage Plan profiles. Alaska also includes a field for a preferred first name on Mileage Plan profiles.
I previously had a binary gender set on my profile prior to Alaska adding these options. After Alaska added these options, the value was reset, allowing me to pick a new option. Making additional changes requires contacting Alaska customer care by phone.
Booking: The same Unspecified (X) and Undisclosed (U) options are included in the gender field when booking a flight.
Check-in: I checked-in using the Alaska app for an international flight with my gender X passport with no issues. I tried validating my passport using the Airside app Alaska recommends, and while the Airside app took my data successfully, the Alaska app didn’t see the validation on its end. My passport was successfully validated in person by a gate agent.
Let Me X Plane 