Click the Settings gear icon to access the drop down menu. Click Mail settings
Click "Send mail from another address"
Enter the Name you want emails to show "From" into the Name field and then enter the email address you would like to show the reply email is coming from
Click Next Step
Choose "Send through Gmail ( easier to set up ) and click Next Step
Click "Send Verification" - this will send an email to the address you're setting up. The email will have a verification number in it that you'll need to enter into the next step.
Enter the verification number into the field and click "Verify"
Check the "Reply from the same address the message was sent to" radio button