Currently we're not able to sync most of DLCs from Steam, because they're not visible on user profile, so there is no way to detect that you have them. You need to add them manually to collection on GG.deals.
We may try to improve this in the future, but that would require different approach for syncing collections. You may vote and follow Add import recognition of Steam game edition & dlc owned & wishlisted to track progress in this matter.
