You first need to add the email addresses that you own to your Account from your Gmail Settings menu.
Click on "Learn more" for full instructions
Once you have added the email addresses you will be able to choose one of these from G Merge / Mail Merge "Send from" section
The recipient will receive emails coming from the alias email address.
You will find your emails sent in the Sent folder of the your Gmail (ie. the account that has started G Merge)