URL Scheme for Google Gmail? WhatsApp and many other services offer URL Schemes (they work like direct links or deep links) for their apps. An example is whatsapp://.
Is there one available for Google Gmail? 
I tried these and they are not working for the latest Gmail release on Android 8.1:

googlegmail:///co?subject=&lt;subject text&gt;&amp;body=&lt;body text&gt;
googlegmail://
gmail://
The URL schema does not necessarily begin with a unique protocol. For example, if you click this link https://www.youtube.com/ with the YouTube app installed, you'll be prompted to open it in the app.

AFAIK the Gmail app doesn't have a "supported links" defined, so you have no chance to make a link open in Gmail.