Android - How can I view SMTP headers in the Gmail app?

Sadly there's no such option in the official Gmail app.

You can try using a third party mail client with more options such as K-9 Mail: http://k9mail.org/

It is not possible to view headers in the Gmail app; you will need to open the Gmail web app in your browser instead.

Using Chrome (mobile, 47.0.xxxx.xx) on Android (5.0.2), for example,

1. Go to gmail.com.

2. Use the three vertical dot menu at upper-right > request desktop site (not sure this step is necessary, but I did it, and it worked).

3. Use the gmail menu (three vertically-stacked horizontal bars) to the left, above the main window.

4. Scroll to the bottom and choose "View Gmail in ... Desktop"

5. Open the email message for which you'd like header information.

6. Click Show Original just above the window containing the message body.

7. The header info will appear.