> I thought gmail doesn't disclose sender's IP address? Or I was wrong? This is not good for privacy.

This is not necessarily a Gmail thing, but just how SMTP works. It's not as bad as you'd think though.

SMTP services log the IP address and/or hostname of the remote host, and the address used by the host to identity itself (known as the HELO address). This is the address of the remote SMTP service (known as the MTA), which isn't typically the IP address of the users computer where the email client runs on (known as the MUA).

Under normal circumstances your email client (MUA) connects to your email service provider (MTA), which then sends the email to the MTA of the recipient. So the IP of your MTA (email hosting service), not your MUA (your computer) is exposed.

For example: if you send an email to a Gmail inbox using MS365, the receiver (the Gmail user) would see only the IP-address from Microsoft's outbound SMTP services.

So unless you run your own SMTP service at home, or attempt to directly connect with the receiving MTA using SMTP, your IP address won't be exposed.