There is another question that asks How to open gmail message links in Gmail Android app?. I don't know how to how to answer the question, but I know of an app that can do this: Google Tasks.
These steps show how to open an email in the Gmail app from Google Tasks.
- Open an email in Gmail
- Select "Add to Tasks"
- Open Google Tasks in Android
- Locate the task that was created in step 2
- Locate the button on that task with a picture of an envelope
- Tap that button
- Observe that the corresponding email is opened in the Gmail app
How does Google Tasks open an email in the Gmail app? Is it possible to do this in custom app (as the linked question asks) or is Google achieving this behavior in Google Tasks using a private API?