This is a bit late but GEDCOM is an acronym for “GEnealogical Data COMmunication” and genealogy programs generate this type of file from the data you've entered into them. It's used to transfer data between genealogical software programs.

If you don't have a genealogy program on your computer to generate a gedcom from, then you won't have a gedcom file to upload to create a tree on Ancestry.com, if that's what you wanted to do. But you can create the tree on Ancestry.com without uploading a gedcom.

Hth.
Jo