Go to GEDCOMs on your My WikiTree Tab.
At the top of the page click on the Download a GEDCOM bar.
This will take you to the page to export a GEDCOM file from WIkiTree. You have a lot variability in what data you can download to your GEDCOM file. Read that page carefully before you start. It is an extremely versatile GEDCOM creation utility....one of the best out there.
I would love to have this as a stand alone program that could read a large gedcom file and export smaller more concise files out of it.