If you mean below all the gallery photos, don't think it's possible. Of course, I've been wrong before... maybe someone else has a solution.
If you mean individual photos within the gallery page, check the help section (link top right of all pages). There's an option which allows placement in the edit page.
ie: Gallery Edit, Advanced: show image captions with thumbnails. alignment=
If there's a gallery doing what you're asking (and I'm misunderstanding) post the link so we can check the CSS.