In your WordPress admin dashboard, go to “Posts” -> “Add New”. You will see the “Format” box in the page editor:

From the available formats, select “Gallery” and you’ll see a new post option appear below the editor:

Click on the “Select Images” button to open the WordPress image gallery. Upload the files or select the existing images you want to include in the gallery post. To select exiting files, press and hold the Ctrl or Cmd key on your keyboard, as you click the items to add to the gallery.

In the post editor view, you can click the “Select Images” button anytime again, to change the choice of images.

Please note that gallery posts will work in every Thrive Theme that supports post formats, but the way in which the galley is displayed on your site can vary greatly, depending on the theme design.