It's simple after you've searched using the available categories from the home page or using the search fields in the header, you'll be taken to the results page and here you can switch between the view you'd prefer; list or gallery.
If you want to view the results in "Gallery View" simply click on the icon in your search results page in the green sticky bar across from "View as:".
If you want to switch back simply click on the "List view" by clicking on that icon.
By default all search results are in list view, however you can switch between both using the icons as above. The view that is active in your search results will be the one that is underlined.