WikiTree only has 1 tree, not millions like MyHeritage. Therefor it is very important to first match existing profiles with those of persons in your GedCom/Family Tree.
As Marion says, a small Gedcom to start with is the easiest to find possible matches. Please make sure all are well sourced and have full locations. That will save a lot of time.
One of our mentors created a tutorial for the GEDCOMpare process that others found very helpful. I hope that you will too.