Q: How should the gender field be completed?
A: For item 7 - Select the option “Male” or “Female” if the individual’s gender is known. Select the option “Unknown” if the individual’s gender is unknown. Leave this item blank if the person is an entity. Item 7 does not create an obligation for a financial institution to collect this data when such collection would be in conflict with the financial institution’s obligations under any other federal law.
This is addressed in the "FinCEN Currency Transaction Report Electronic Filing Requirements" guide, Attachment C.