Usually, Gallery image issues happen when eBay was not able to fetch the self-hosted image provided in the request; because the Gallery image is generated at the time of listing. If the image was not accessible (here you can find possible reasons), the gallery image will not be generated for that item. In this case, you'll see an eBay error "Your listing is successful but there's a problem with your Gallery picture".
There are a few methods to resolve this problem:
1) Stop that item and list it again via M2E Pro. Important: You should use List action, NOT Relist. As for Relist action, old (before stop) image settings are used.
2) Use this tool to fix Gallery Image.