Set the Custom Location to your Google Drive for Desktop Zotero folder that you created. For example, "G:\My Drive\Zotero_Downloads".
If you marked up comments or tags or notes on the pdf file from Zotero Application, the pdf file in your Google Drive folder will not be updated automatically. But the mark-ups are preserved when you open the Zotero PDF reader.
To save the mark-ups on your pdf file in the Google Drive folder, you need to take a few extra steps:
Then the PDF file in your Google Drive Zotero Downloads folder will be replaced with the one with those comments, tags, and notes.