Gallery page need image as the main content. You can upload it at feature image in Gallery page (at the admin panel > gallery). If you do not upload your image to that page, page cannot show your gallery image. Make 1 gallery page for 1 image. After you make gallery pages, the pages will show in the work page. You also must fill the category for gallery page, because gallery will turn to filtrable in work page.
In post gallery you will find Feature Image section. Click the image and then will display an option like this.
After that fill the caption form with your words and set featured image.
Remember: Each photo on Bow theme can only appear in one category,
if you put one photo to more than one category, your Site / Work Page may not work so well. That's Bow theme built for.