I am using Yet Another Mail Merge with Google Spreadsheets and Gmail and it works just as expected. But I am also trying to add a Google Drawings into the email body, so I would have more flexibility to have text areas (I have not found a way to enter a text area in Gmail).
The email merge works out of email draft. So I am trying to have a text area (where I can add the spreadsheet fields) on top of a image then export to email.
Once I copy to web clipboard and try to paste onto email body, the text box slides down the picture. And it looses some details.
In summary: have a text box on top of image inside the body of email. Text box to have spreadsheet details. Everything works OK in Google Drawings, but it does not seem to be able to export to Gmail.