The use of fonts is currently limited by support in e-mail clients. We advise using web-safe fonts to ensure your signatures are displayed the same way in different e-mail clients.
Gmail doesn't support methods for importing or linking fonts from external sources; it strips header styles, and there's no way to inline custom font declarations. Email has so many more constraints than the web because of the many and different ways the HTML is consumed and altered by clients.
If you still want to use a custom font despite these ramifications, please go to Settings and add the font of your choice by typing its name into the corresponding field.
Don't forget to add a fallback font - if the recipient of your emails won't have your primary font available, a fallback font will be used instead.
After you save the changes, the font will become available in the editor as the first choice in the Styles menu where you can apply it to the signatures.
If you decide to switch to a different custom font later, you need to confirm the Style change by clicking on Force style.
If you are not using Styles, you can still pick the font from the offer directly in the editor.
Since you can always have only one custom font available, you will need to use this option if you plan to utilize two or more custom fonts in your signatures. Modify your template with the first one and add the second one in Settings again. The former won't be available in the editor now but it will stay in the code of the signature.