The value proposition behind google hosted domains is falling.
Whoever is writing this seems to have absolutely no clue how AI works.
Given that Google is clear about the fact that they don't train on your emails, the worst that could be happening here is that... within the scope of your account they maintain an extra index or two, or... additional synthesized data, in addition to the many indexes that they already maintain over your email.
Of course reading generated text remains the sole responsibility of the user before clicking "Send". We all know that reading drafts can happen more or less carefully, especially when being in a hurry.
The question isn’t really about how AI works. It’s about how Google (the company) works. Do their actions match their stated intentions? Which is really a question of trust. Are they incentivised to lie? Yes. Are they likely to survive a disclosure scandal? Facebook’s experience inclines me to believe yes.