If you're using email to communicate with humans, you are using Gmail since your counterparty is almost always using Gmail. Gmail and their ad scanning is unavoidable in practice when using email so your only real recourse is to use a different communication protocol.

Gmail is reasonably secure in situations that actually occur frequently. No other providers are. But even Gmail is optimized for adoption and monitization over security, so it's security efforts only go so far, particularly when interacting with other email providers.