Whether you upload a GEDCOM or not, you add people to WikiTree one at a time. Using the GEDCOMpare tool can be handy if you have a lot of dates and placenames already entered that you don't want to type again, and/or relationships already figured out that you don't want to link up all over again, but my experience is that the process kills link URLs and mangles sources, so for some people, manually copying-and-pasting from an offline tree may be easier.
If your family is likely to already have members on WikiTree, then the GEDCOMpare tool is also handy for identifying them sort of all at once instead of piecemeal. However, you will not be able to make definite "match/not a match" decisions unless you have reasonably complete details on your offline tree. Therefore, if your offline tree is mostly just names, I suggest not bothering with an upload. Enter people one at a time, based on your offline tree, but using this as an opportunity to flesh out your data on both WikiTree and Family Tree Maker.