Here's what you can do to change the default Font in Gmail while composing a mail:
1. Go to "Settings" at the top of your Gmail page.
2. Select "Labs", as shown in the image below:
5. Now, navigate to the "General" tab and scroll down to "Default Text Style"
6. From the drop down under the letter "F" select your preferred font and then "Save changes" again as we did earlier:
And Voila, you're done.