Can I move old files from my Mac into Google Drive to free up space?
Short answer: Yes. Create a transfer from your Mac folder to your Google Drive account and choose move — the files go to Drive and leave your Mac. Large amounts take a while because Google limits upload speed.
Yes. Connect Google Drive, then create a transfer with your Mac folder as the source, your Drive account as the destination, and choose move. The files end up in Drive and leave your Mac, freeing the space.
For large amounts, keep in mind Google limits upload speed, so it can take a while; it carries on in the background. If you want to keep a local copy too, choose copy instead. See Data Transfers.