Open the Spreadsheet in your web browser. In the URL at the top, you should see something like this https://docs.google.com/spreadsheets/d/1-ABC0123456789/ . The string of letters, numbers and symbols (1-ABC0123456789) is your Spreadsheet's ID.
Why does the Job creation page take a while to load?
To get the Spreadsheets and their Sheets (Tabs for Excel users) for you to choose from when you create Google Sheets Jobs in Gravity we have to talk to Google's Drive API.
The problem is, although that API gives you some control over what Spreadsheets you want, if you don't make use of those controls and have access to dozens of Spreadsheets, the API takes a while to get them.
So, we have exposed some of those controls to you as optional settings.
By far the best option to speed things up is setting Spreadsheets (optional) which restricts your Google Sheets Source to a specific set of Spreadsheets.
The next best option to speed things up is excluding Spreadsheets in Trash or Team drives. By default, these optional settings are unchecked for this very reason.