True ETL Tools
If you need more than just to copy data from Gmail to Google BigQuery, you may use Skyvia's powerful ETL functionality for Gmail and Google BigQuery integration. Skyvia's Import tool will correctly extract Gmail data, transform it, and load to Google BigQuery when Google BigQuery tables have different structure than Gmail objects. Besides, if you ever need to load data back from Google BigQuery to Gmail, Skyvia Import allows loading data in any direction, supporting Reverse ETL scenario.
For more complex cases, you can use Data Flow and Control Flow tools. They are designer-based solutions for creating complex data pipelines, involving not only Gmail and Google BigQuery connectors, but any number of connectors in one integration. With these tools you can automate data workflows, involving custom logic, complex multistage transformations, combine data from different sources, load them into multiple destinations at once and many more.