This is not a good heuristic, for several reasons.
First and foremost, despite widespread fetishization of things like end-to-end encryption, real world software rarely differentiates its security based on superior cryptography. Most security vulnerabilities occur at the endpoint level and are not even technical vulnerabilities, they're just successful, targeted phishing campaigns. The next most common set of vulnerabilities are in mundane software, typically in the infrastructure and peripheral logic. These are due to developer ignorance, misconfiguration or sometimes both. From there we have crypto implementation flaws, which are typically due to software engineers' fascination with implementing their own cryptography libraries or taking strange liberties with existing libraries that deviate from the explicit or implicit intentions of the original author. Finally, on a peak so remote and small it's scarcely visible from the ground, we have actual cryptography design flaws, where someone literally rolls their own crypto at the conceptual level and deploys it.
Second, email is a fundamentally antagonistic medium when it comes to end-to-end encryption. If you are actually concerned about your privacy and you're discussing something that warrants extreme care, email is the least user-friendly and most error-prone method of going about it. You should consider a synchronous medium with forward security if possible, probably based on a well-known and well-audited messaging protocol (to avoid a flame war I'm not going to suggest any particular one - do your research).
Third, if your threat model is honestly the US government, you need to significantly revise your opsec entirely if you're realistically considering Protonmail. Regardless of its actual security, you're (implicitly) saying that you trust Protonmail to be capable of withstanding the resources of a motivated three letter agency with an armada of security compromising tools at its disposal. Why trust a third party at all then? What makes you think the servers being in Switzerland is going to help you if you distrust the government this much? By all means, don't use Gmail either, but then Protonmail isn't really a coherent security measure either - again, use a synchronous messaging platform, or develop the opsec needed to consistently use PGP correctly on your own.
Given the foregoing, if your adversary is actually the US government, neither Gmail or Protonmail are effective strategies, and if your adversary is not the US government, Google's security team is vastly more qualified and has overwhelmingly more resources at its disposal to secure its email infrastructure.