I created a carousel in the page and then dragged it into the header section.

To insert an image carousel to your Site:

1. In the right sidebar, click Insert > Image carousel 

2. Select images to upload or select from already uploaded images. 

3. Drag images to reorder as desired. 

4. Hover over the image to delete or add alt text. 

Read More:

https://gsuiteupdates.googleblog.com/2019/05/create-img-carousel-sites.html

 After adding the carousel you can drag it into the header. Note: The carousel cannot be made full-width.