When your content is not visible in Gmail but is in other email clients, it often is because some styling is missing. When one property has no value in the style tag, none of them will have a value in Gmail. The text is there but because it has no size, it will not be visible. Make sure that all text styles have a value in Skins (account settings) and projects are not overwriting the account settings with an empty value (this can happen when you delete the value with the keyboard and not with the trash can icon).
In the screenshot below, you can see the same email - left opened in Apple Mail, right in Gmail. You can see that the text is even in Gmail, but it has no styling. Apple Mail shows the text in the client's default font.
In the screenshot from the editor, you can see that the font-family is missing and the text is displayed in the browser's default font. The font-family is missing because it has no value in Skins.
It should be: