Google recently added confidential mode for Gmail that allows the sender to restrict how their email gets by the recipients including forbidding printing, etc. However, it looks like that Gmail for android still allows screenshots to be taken for such emails. To replicate, try the following:

1. Send a confidential email to another Gmail user.
2. Open the email, press Power + Volume Down. A screenshot will be taken.

The underlying reason is because the Gmail app is not using “FLAG_SECURE” for such screens (more information on FLAG_SECURE can be found in our earlier blog post). By contrast, many Android apps with higher security requirements use it. For example, Chrome for Android prohibits screenshots when browsing in anonymous mode on Android.

We did file a bug report with Google and here is their response:

We can not prevent someone from taking a photograph of their phone, using a second camera for example. Preventing taking photos of the content is not part of the feature.

References:
Google bug report # 112838515