Syncdocs takes care not to make duplicates, which is a common problem for Google Drive users. You don't need to upload everything again. You can just point Syncdocs to your existing Google Drive synced folder at installation, and it will see that the files are the same and not re-sync them. Just make sure your directory hierarchy is the same.
You can also copy the files from one PC to another over a LAN or USB rather than copying to Google and back.
A more technical explanation: Syncdocs checks the local hash of the file contents with those on Google. If they are the same then it does not re-sync them.
Don't run Google's Drive client and Syncdocs on the same folders at the same time, rather exit Google's client.
EDIT: It is not only for documents. You can sync any sort of file. There are some special Syncdocs features for document collaboration, say between MS Word and Google Docs, but it can also just sync/upload/download files.