Gmail has a vested interested in keeping any knowledge it gains about you secret - it's competitive advantage is knowing more about you than anyone else does.
ChatGPT's strength is its ability to clearly communicate the knowledge it has (including training data it gains from people it interacts with) to give you good responses.
I am still not seeing a huge threat to be honest. This is not how attacks are done. OpenAI also has vested interest to keep your data safe and is strongly linked to Microsoft.
Most corps, companies store a lot of internal data with corps like Google, Microsoft, Amazon and others.
Gmail has a vested interested in keeping any knowledge it gains about you secret - it's competitive advantage is knowing more about you than anyone else does.
ChatGPT's strength is its ability to clearly communicate the knowledge it has (including training data it gains from people it interacts with) to give you good responses.