Task: I want to reply to Amelia Miller's email that "Sorry to say, but I'm not able to come to this event" in the Gmail app.
Steps:
Click on the Amelia Miller to view the reply option of this email.
Click on the Reply option to reply this email.
Type Sorry to say, but I'm not able to come to this event. in the body of this email. 
Click on the send button at the top right corner of the screen to send the reply of this email.