Perhaps due to today's outbreak of a widespread phishing scam, or simply by coincidence, Google is rolling out enhanced anti-phishing security checks in Gmail for Android. When users tap on a suspicious link, the above warning will now appear.
Judging by the page design, this feature appears to use Google's Safe Browsing database, which catalogs sites known to host malware or trick users into giving away information. Ironically, this wouldn't have done anything to prevent the recent phishing outbreak, as the link in that email redirected to a normal Google login page.
The change appears to be part of Gmail for Android 7.4.23, which you can grab from APKMirror right now, or wait for the Play Store to roll out the update to you.
- G Suite Updates