As of version 10.5, it is now possible to add custom fonts into the New Drag and Drop editor. In order to do so, you will need to open the editor for your email, select the Global Styles tab, and select the "Add Custom Fonts" option at the bottom of the tab.
This will open the "Add Custom Fonts" window.
Add CSS font URL: This is the URL of the font that you would like to load. For acquiring this URL from Google Fonts, you would need to open your desired font and select the "Select this style" option.
This will open a window with options for the font. You will then want to click on the "Embed" tab and copy just the URL provided from the <link> area as shown below.
This should give you the appropriate link needed to load the font. Afterwards, the font should then appear in your font drop-down menu under the 'Custom Font' section.
Preview Name: This is the name that will appear for this custom font in the font drop-down menus.
Select fallback font: This is the font that will be used if the custom font is not able to be loaded by the recipient's email client.