In a sense, yes. Email contents in gmail are technically accessible to Google. But they are protected like hell via a bunch of dedicated systems that make it very difficult to access this material without an explicit auditable ticket associated with helping that user with some problem and permission to access their gmail contents. Attempts to circumvent this will get people fired.
This does rely on you trusting Google to implement and use these systems. The question is whether you trust a major tech company or whatever Mastodon server owner more to not peek at your DMs.
I know yahoo employees read email, I got my account back once by telling them recent subject lines.