Create GEDCOM: GEDCOM starting from Abraham Lincoln
I did find this online but have no idea how this would work, whether it would deliver unto you a gedcom ...
Usually gedcoms are uploaded from outside WikiTree onto WikiTree. I suppose there may be a way for a WikiTree PM to generate a gedcom but I've not read anything about that happening.
Anyway, if you decide to test out that site to see if you can get a gedcom from it, let us know how it worked out, please.