A perfect example would be a photographers portfolio website. They may have different pages for different types of photos such as nature, cars even maybe something small such as street signs. Having a car page would mean, no images of nature should be in the gallery of the car page, unless the photo over laps each category. Then the photo could be an exception.
Similarly to adding almost anything to your site, to add a gallery, you use the add element again. Each gallery you will see will be different from each other. From professional looking galleries to the less so professional. Wherever you choose to place a gallery on your site, always make sure the images are appropriate for the page or your website.