yeap, better clarification of your concern
with Sync tool you just sync (rsync) your files and folders between two storage environments, in this case between NAS and Google Drive (single or two way).
There isn’t any conversion included.
But there is a workaround how to do it in my everyday's case (I have G-suite):
1. Deploy Google Drive File Stream:
With Drive File Stream, you stream your Drive files directly from the cloud to your Mac or PC, freeing up disk space and network bandwidth. Because Drive files are stored in the cloud, any changes you or your collaborators make are automatically updated everywhere. You’ll always have the latest version.
You can use Drive File Stream on these operating systems:
Windows: Windows 7 and up. Windows Server 2012 and up.
Mac: El Capitan (10.11) and up. For High Sierra (10.13) or newer, follow these steps
Windows only: Internet Explorer 11, Microsoft Edge
Mac only: Safari
With this feature you can open/edit/save any original Google docs synced by NAS to your local computer without necessary conversion. OFC you can convert all doc to Office format.
This test was prepared for your better understanding from my W8.1 environment, same is in W10, then same must be in MacOS: