Use a reputable service like Have I Been Pwned to see if your email has appeared in known data breaches.
If you are concerned your information might be in such a file:
In this context, "private" is often a marketing term used by data brokers to suggest the list is fresh, unique, or hasn't been widely leaked yet, making it more "valuable" for malicious actors.
The term "lines" usually refers to text entries formatted as email:password . These are used by automated tools to attempt unauthorized logins (credential stuffing) across various platforms.
Since many people reuse passwords, a "Gmail line" might grant access not just to an inbox, but to linked bank accounts, social media, and cloud storage.