Based on a url like
https://plus.google.com/123456789012345678901 # dummy number
How I can find the gmail username?
Based on a url like
https://plus.google.com/123456789012345678901 # dummy number
How I can find the gmail username?
You can't. For privacy reasons, having someone in a circle or knowing their Google Plus id should not be enough to get an email address.
If you are developing an app or script, you can use Oauth 2.0 and request the userinfo.email scope.
For details, see https://stackoverflow.com/a/7836385/1354939
This does require that you configure a Google APIs Console project and create a OAuth 2.0 client ID.