Hi Thomas, and welcome to WikiTree!
There is an ability to upload a GEDCOM of your data, but unlike other sites, this information is not automatically imported. Instead, we use a process called GEDCOMpare in which your GEDCOM provided data is checked against the existing profiles here on WikiTree. If you find matches to profiles, you can easily add any new information (sources) to the profiles and mark them off your list. Any data you have that does not match existing profiles can later be added one-by-one.
This is very important to us, since WikiTree is a shared family tree - one profile for every individual. We work to prevent duplicates, and we all collaborate on our ancestors. We are all cousins here!
One thing you may find though, is that many members like to add profiles one-by-one, sourcing and documenting the relationships as they go, rather than messing with GEDCOMs. This allows you to verify your data, try to connect to existing profiles here, learn the WikiTree markup (and how to interact with profiles), etc.
If you have any specific questions, just let us know!