The traditional way to sync Google Drive with Dropbox free is copying files from Google Drive to Dropbox, to achieve this, you need to download from Google Drive then re-upload to Dropbox. This is manual way, not automatic way.
Then, how to sync Google Drive with Dropbox automatically? You should try MultCloud. MultCloud is a multiple clouds manager supports Google Drive, Dropbox and other 23 cloud services. The core function of MultCloud is moving files across cloud drives, so backup Google Drive to Dropbox is easy. If you want to sync Google Drive with Dropbox automatically, MultCloud’s scheduled transfer feature will help you a lot.