Gallery Images in the Sandbox Gallery Images can be specified in both SYI (Sell Your Item) and Additem in the Sandbox. AddItem will return the appropriate (Sandbox) fee structure for Gallery, Gallery Featured, etc. GetItem will return the GalleryURL and Gallery tags as specified.
The Gallery Thumbnail Batch Job in the Sandbox is not currently supported. This implies that your Gallery Image will not be displayed in the Sandbox Search view which display "Gallery Images". The link to your Gallery Image Thumbnail will not be valid and there is not currently a manner to display your Gallery Image Thumbnail in the Sandbox.
For the time being, we suggest verifying the existence of the Gallery tags and URL using GetItem. This will reproduce the current behavior on eBay.com. The only difference between the Sandbox and eBay.com will be the (web) existence of the Gallery Thumbnail for listings that were specified with Gallery or GalleryFeatured.