No it does not, it would be irresponsible to do that on private data. There's a very clear line between data posted publicly and data held privately, especially in terms of copyright. I doubt it will ever be default opt-in for something as sensitive e-mail and docs.
One exception to that is scanning for CSAM and Terrorism and DMCA. And with DMCA, it's automated based on file hash, and you still maintain access to your files, you are just limited from sharing them. Ads in gmail aren't based on content, but other online activity while logged in.
I think the other exception to that is smart compose. AI models do use email content for training data, but the output of those are strictly for use locally while writing emails. I imagine it's also siloed per user.
EDIT:
Not a google employee, I apologize if my assertions seem too strong.
EDIT2:
https://en.wikipedia.org/wiki/Federated_learning
"We have always maintained that you control your data and we process it according to the agreement(s) we have with you. Furthermore, we will not and cannot look at it without a legitimate need to support your use of the service -- and even then it is only with your permission. Here are some of the additional measures we take to ensure your privacy: (reference: GCP Terms).
In addition to these commitments, for AI/ML development, we don’t use data that you provide us to train our own models without your permission. And if you want to work together to develop a solution using any of our AI/ML products, by default our teams will work only with data that you have provided and that has identifying information removed. We work with your raw data only with your consent and where the model development process requires it. "
https://cloud.google.com/blog/products/ai-machine-learning/g...
https://support.google.com/mail/answer/6603?hl=en
https://arxiv.org/abs/1906.00080
https://ai.googleblog.com/2017/04/federated-learning-collabo...