While initially there where thoughts that a single Resource should be used to collect details of ones Gender Identity, Recorded Sex or Gender, and Pronouns, there are reasons/use cases that would support the use of both the Patient Resource and Observation Resource. 

The use of the Observation Resource would support providing the source and method for the collection of personal characteristics (i.e. Gender Identity, Recorded Sex or Gender, Pronouns) over multiple encounters that may change over time.  While the need for an institution to have a single instance to identify Gender Identity, Recorded Sex or Gender, and Pronouns would support the use of the Patient Resource to contain the data.

For these reasons, I recommend both the use of the Social Determinants of Health IG proposed[ Personal Characteristic Observation|https://build.fhir.org/ig/HL7/fhir-sdoh-clinicalcare/draft_specifications_for_personal_characteristics.html] as well as the Patient Resource.