S/MIME Certificate Verification Issue with Apple Mail for Gmail Account on MacBook Pro

I'm facing a problem with my Gmail account (let's say: myemail@gmail.com) when sending emails through Apple Mail on my MacBook Pro. The recipients are getting warnings that the authenticity of my S/MIME certificate cannot be verified, even though I am the authentic sender of these emails.

The warning message details are: "Mail was unable to verify the authenticity of the S/MIME certificate provided by 'myemail@gmail.com'. Messages signed by this user may be coming from a different source."

When I inspected the certificate in my MacBook Pro's Keychain Access, it showed that the certificate hasn't been verified by a third party. As I understand, this implies it's a self-signed certificate, which isn't inherently trusted by my system.

Here are the steps I've taken so far:

1. Inspected the certificate in Keychain Access. It's not expired, but it's not third-party verified.
2. Considered manually trusting the certificate, but I'm wary due to potential security implications.
3. Thought about replacing the certificate with one from a trusted Certificate Authority, but I'm not clear on how to achieve this specifically with Gmail and Apple Mail.

I have not made any changes yet because I want to approach this situation correctly. I would greatly appreciate any guidance on how to resolve this issue, whether it's obtaining a new, trusted certificate, or if there's another solution I should consider.

Affected device: MacBook Pro

Software: macOS (latest version), Apple Mail (latest version)

Thank you in advance for your assistance!