When creating a display ad, Ad gallery lets you:
- extend a Display campaign’s reach to Google search partners
- automatically adjust image content based on campaign performance
- choose from existing design templates
- automatically create a display ad from existing ad text
The Ad gallery, formerly known as the display ad builder, is an ad-creation tool that offers various display ad formats in different categories, such as image ads, dynamic ads, Lightbox ads, and video ads, in any of your campaigns on the Google Display Network. Here are some benefits of using the Ad gallery: Differentiated products and services, More effective campaigns, No-cost ad templates, Customizable ad styles, Industry-standard ad templates.
Read more here: https://support.google.com/adwords/answer/1722134?hl=en