Create a gallery with the "Gallery Theme" set to "Gallery Image 2", add an image and put the following payload in the "Image Description" field: <svg onerror=alert(/XSS/)>
Save the image and gallery and view a post/page where the gallery is embed to trigger the XSS

The "Image Title" field is also vulnerable, with a payload such as "><img src onerror=alert(/XSS/)>