If you would like to use the default font that is seen in the Gmail interface for your signatures, you can achieve this by simply removing the font from the dropdown menu in the signature editor.
If a font has already been set for your text, you can remove it by selecting the font again in the dropdown menu.
Alternatively, you can switch to the HTML Editor (available on the right sidebar) and remove the font from the CSS style selectors in your HTML. For example:
- Change: <p><span style="font-family: arial, helvetica, sans-serif;">This is default font</span></p>
- To: <p>This is default font</p>