A Portrait Gallery is a Gallery where all App Preview Videos /Screenshots are oriented in the portrait format.
A Portrait Gallery in the Apple App Store will always display Videos before Screenshots. Only two Videos /Screenshots are visible on both the Product Page without interacting with the Gallery. In the Search Results Page, only three Gallery assets will appear. Either one Video and two Screenshots or just three Screenshots will be visible.
In a Portrait Gallery in the Google Play Store, two-three Screenshots are visible on the Product Page (depending on device) without interacting with the Gallery.