Our suggestion is to use Provenance, with a target reference to the Patient resource, and a targetElement referencing the gender identity extension.  This approach is not unique to gender identity, however, as the same approach can be used for other demographics like race and ethnicity.   We will update the definition narrative for the individual-genderIdentity extension with the following language:   An individual's personal sense of being a man, woman, boy, girl, nonbinary, or something else. This represents an individual’s identity, ascertained by asking them what that identity is.     In the case where the gender identity is communicated by a third party, for example, if a spouse indicating the gender identity of their partner on an intake form, a Provenance resource can be used with a Provenance.target referring to the patient, with a targetElement extension identifying the gender identity extension as the target element within the Patient resource. https://build.fhir.org/extension-targetelement.html