Usually, Gallery image issues happen when eBay was not able to fetch the self-hosted image provided in the request; because Gallery image is generated at the time of listing. If the image was not accessible (here you can find possible reasons), gallery image will not be generated for that item.
There are a few methods to resolve this problem:
1) Stop that item and list it again in m2e pro listing. Important: You should use List action, NOT Relist. As for Relist action, old( before stop) image settings are used.
2) Use the following tool to fix Gallery Image: http://cgi5.ebay.com/ws1/eBayISAPI.dll?DisplayGalleryImagePageV3