I found this on Lifehacker, but it can be a pain to search their site, so I’m posting it here, with a tweak I found.
Digital Inspiriation has a great tip on how to do mail-merge in Gmail. They have a Google Doc you can use in conjunction with a contacts group in Gmail, to email the whole group, but with each individual person’s name at the beginning.
I wanted to tweak it slightly though and have it just display a first name, rather than their full name. I found this tweak, where you just edit the script’s code to say “getGivenName” instead of “getFullName”! It works perfectly.