As lloyd notes below, the basic patient gender datatype is defined as the Code to simplify the base patient resource to a more static set of values. Many systems use their own values and require mapping to other values anyway (govt's don't define the same set) so loosening this binding wouldn't improve interoperability. This has been discussed at length in other tracker items and Zulip. If you need more values than in the base set, you can extend the other code value. And for non administrative gender usecases, the observation should be used (or other extensions on patient for things like gender at birth or jurisdictional variants that are additional to the adminsitrative gender).