The reason the font will look different in some email clients, such as Gmail and Outlook, is because not all email software supports custom fonts (including Google Fonts.). So it's not a limitation of our email templates but a limitation of the email software the email is being viewed in.
In the ones that do support it, your custom font will be shown, and in the ones that don't, we've picked "fallback" fonts that match your font as closely as possible. Of course, if the fallback font doesn't match the custom one at all, please let us know so we can see if we can find a better fallback for it.
If you want everyone to see the same font, no matter what, then you can use one of the following:
Arial or Helvetica
Courier New
Georgia
Impact
Trebuchet MS
Verdana
Tahoma
Times New Roman
Personally, I would use the custom font that matches your site the closest and then allow the fallback fonts for people that don't have a supported device, as they would never know what the custom font version would have looked like anyway.
Note: The font you've chosen in the app editor might not appear in the Shopify email preview. However, it should display correctly in actual emails when viewed through a supporting email client.
Have any more questions? Feel free to contact our friendly support team using the message icon on the lower right corner of this page.