Gmail:

• Gmail removes <style> tags that include an @. To solve this wrap your @import or @font-face in an @media
  
  Example: @media {
  @font-face { Email Font Here
  }
       }
• Gmail Ignored My Image - This can happen when you have spaces in your image file. While other email clients will render this image properly Gmail will not render this properly and fail the image to load
• Gmail stripped out all of my code - If you’re writing all of your code in the head of your template use a CSS inline tool or hs-inline-css