How to change and set the default font in Gmail?

Step 1: Log in to Gmail from your Web browser(Google)

Step 2: Open the settings menu in the upper right-hand corner and choose Labs.

Step 3: Scroll down to "Default Text Styling" in the list, and enable it.

Step 4: Click on the General tab of the settings and find "Default text style."

Step 5: Adjust the font-face, size, style, and color of the text to your liking.

If you change your mind, you can always set it back to the original font, or choose another of your liking.