Have you now noticed that when you receive an email in your Gmail, sometimes it shows the images automatically, but sometimes you have to click on display images, for them to show?
Well, it has been a year since Gmail said that if you sent at least 2 emails, to the address that is emailing you, they will automatically download it.
As a suggestion, I strongly recommend that you monitor your sends in the Reports and Statistics Section of Emailbrain, to see if your Gmail emails are being opened without a problem. Also, it will be a great idea to use the link that Emailbrain provides at the top of the emails to reinforce the option of unsubscribing, if your client would like to do so. Furthermore, ask your clients to add your email address to their contact list and this will guarantee them receiving your emails, without getting filtered by Gmail or any other email provider.