Thank you for your reply.
I'd tried that, but the version of GAM that I'm using doesn't accept a URL as a file identifier.
I have, however, devised a workaround. What I did was to create a new Google Sheets file, and use an importrange formula to import the data from the tab in the original spreadsheet. I can then use GAM to download the data from this new spreadsheet, and as this new spreadsheet has only one tab, there's no issue with making sure that I'm getting the right tab in my download.