When we use Gumlet, the image SRC attribute is replaced by a pixel which is fine with the front-end. Nonetheless we see the same pixel in src attributes of RSS feeds which doesn’t make sense and don’t allow us to display the feature image correctly. In that case only the pixel is displayed. To check this issue, visit domain.com/feed of any WP with gumlet installed.
We already ignore feeds and you can check it on our test deployment: https://wordpress.turingiq.com/feed/
Do you face any issue with your deployment? If yes, it would help if you can provide feed URL so we can check and resolve this.