Based on a url like
https://plus.google.com/123456789012345678901 # dummy number
How I can find the gmail username?
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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.