android-app://com.google.android.gm

What it is

com.google.android.gm is the Android package name of the Gmail app. The android-app:// scheme is how Android identifies a native app as the source of a visit.

Why there's no full URL

Apps aren't web pages, so there's no page URL to pass along. Android lets the app identify itself with an android-app:// URI built from its package name, and that's all the referrer contains.

What traffic it represents

Visits where someone tapped a link inside the Gmail Android app and the app (or the browser tab it opened) reported itself as the referrer.

Sources

Gmail - Apps on Google Play

Gmail - Google Play

it can also use the android-app: scheme to identify a native application that it came from.

Intent.EXTRA_REFERRER - Android Developers

otherwise, if known, an android-app: referrer URI containing the package name that started the Intent will be returned.

Activity.getReferrer() - Android Developers

You can set your app as the referrer when launching your Custom Tab. This way you can let websites know where their traffic is coming from.

Customizing the UI - Chrome Custom Tabs