You should have "Google Photo" on the phone as well, although you may not be actively using it. To see if the photos are still there and accessible, try using that app. It can be phone under P for Photo. If they still are not found, I would suggest hooking the phone up to a computer and seeing if you can locate them via the computer.
Also, is it possible that you sent them to the cloud? or another similar place?
Lastly, were the pictures on the phone itself, or on an SD card? If on the card, it is possible the card is damaged or dying. In that case, the photos may be lost.