Hi @mindc,
Generally yes, if the connection has no triggers in Microsoft Flow, then it can't be used as a trigger in Microsoft Flow.
For the flow OneDrive ->Google Drive, the flow created here is not file synchronization. The flow created is more like copy files, for files which already exist, it will overwrite.
You shoud have noticed that when specify the file identifier, file content is selected as a whole.
And this is not bidirectional.
For example, if you have added a new file in Google Drive, this file won't be added into OneDrive, with this flow configured.
Regards