Now you can zoom videos on Google Photos with pinch to zoom and double-tap. It turns out that double-tap is adding a little bit of zoom effect while pinch to zoom can bring you very close. Zoomed videos are also playable so you can watch the whole vid and scroll the navigation bar while zoomed. This is a server-side feature that was enabled for everyone.
Google Photos app doesn't have a beta version available but it often receives new features via background in-app updates.
Source: Android Police