Google redesigned its Task app after getting rid of its standalone service and now, the company has ported this feature into Gmail on Android. So, if you’re using the improved app, you can now add emails into it. You just need to tap on the overflow menu (the three-dot menu) and click “Add to Tasks.” You’ll get a notification if an email has been added successfully, and whether you want to jump directly into the Tasks app to view this new addition.
Source: Android Police