Hi Samuel
It is not possible, sorry.
Sync only works one way.
You could setup a second sync the other way round, but then the system would not know which files to keep. So unfortunately a bidirectional sync like you get with downloading the GDrive App is not possible.
btw: even if it were possible, TrueNAS access GDrive over an API, so performance is very, very slow. See this thread: Dropbox Sync time too long
What you could do is setting up a VM that hosts the GDrive App and share data over that VM. One big disadvantage is that you are forced block storage for that VM with all the disadvantages that come with block storage over datasets.