You can import a Gedcom but I do believe you will have to go through them individually afterwards to check for matches to existing profiles before they are added to the tree. Also, size of Gedcom is limited. You won't be able to just import, say, 3000 people in one go. I also think you won't be able to import profiles with a birth date earlier than 1700 (? not sure of that date). Someone else may be able to advise on the specifics of the above. There is also the expectation that profiles be spruced up afterwards to remove any autogenerated gedcom text that has no function on Wikitree and bring them up to Wikitree standard, ie ensure that they include primary sources.
N.B. Size of Gedcom was limited because in the past, people used to import massive trees and just leave them there untouched as though Wikitree was just a repository, which led to a large number of profiles needing attention and an army of volunteers was needed to get the profiles up to standard. It also led to large numbers of duplicate profiles, hence you now have to check them off yourself, in order to avoid duplication.