Google has added a rotating video option back to the Google Photos app after having been removed from an earlier version. Google removed the feature from Google Photos when they separated the app from Google+ Photos, although it appears that Google has decided to bring the feature back as of the newest version of the Photos app, which is version 1.25. If your Google Photos app is sitting on the latest update, then you can access the video rotation option from any of the videos that are found in your camera folder, although this only applies to videos which you have used the smartphone camera to record.
If you're in the camera folder you can tap on any of the recorded videos that are there and then by selecting the edit option, you'll be taken to the screen that has the slider bar for trimming your videos. Just below this is the rotate button, and tapping it once will rotate the video 90 degrees. You can continue to hit the rotate button until it comes full circle back to its original position as well, so if you accidentally hit the button too many times you can simply tap it a few more to get back to where you wanted it.
While this is a fairly minor change, it's one which is sure to be appreciated by some of the users out there. This follows the version 1.24 update from back in July which brought improvements to the image crop feature, by way of an updated cropping tool. Another prior update from a month before introduced the slideshow feature in Albums. If were quite fond of the rotation option for recorded videos back when Photos was still part of Google+, then you'll likely be happy to see that's once again available as an option. You can update to the latest version of Google Photos by simply going to the my apps section of the Play Store and updating the app. It's also possible that this is something which Google flips the switch on for, meaning users may not have to do anything to see the rotate feature pop up besides wait patiently.