However, if you want to get started earlier, you can start with the Gravity Forms logging. You can enable logging by going to Forms > Settings > Logging. Select: On, then [Save Settings]. By default this will enable all logging on your site. Documentation for Enabling Logging: Logging and Debugging - Gravity Forms Documentation
After enabling logging, test the form. After testing the form, return to the Forms > Settings > Logging page and view the Zapier log or the REST API or Gravity Forms log and see what is going on behind the scenes.