On your profile page, you should see (at the top) an 'Add' link. Hover over that and you'll see a link for 'GEDCOMs'. Click on that. Then you can read about importing one and then click on the link entitled 'Click here to upload your GEDCOM'.
Be sure to read the important information, before you try to upload one, about uploading a GEDCOM. If you've tried to upload one and it doesn't work, it might be due to one of these things that aren't allowed:
- GEDCOMs with more than 2,500 people cannot be imported.
- GEDCOMs with more than 100 people and no sources cannot be imported.
- GEDCOMs with more than 500 suggested matches cannot be imported.
I personally recommend that you start with a small import at first just to see how it goes. You should also be sure that you have at least one source for each profile you're wanting to import. Also, have you searched for numerous of the older profiles you're wanting to import to see if they already exist on Wikitree?