It could be called by a different name such as "album" or may be hidden if using a launcher.
Open your camera app and see if there's an option to open your pics from there.
Also restart your phone to see if the app appears.
You could always download an app from the Google Play store if your phone didn't ship with one. I'm partial to Google Photos, which integrates with your Google Drive so you can back up your photos to the cloud if you so choose.
But make sure you didn't just remove a default gallery app from your home screen. Access the app drawer (the method depends on which phone you're using) and look around for Gallery, Photos, something like that. If it's there, long-press and drag it to the home screen of your choice.
Look for Photos, it's replaced Gallery in most phones. (It may not be on a home screen by default - as djrakowski said, you'll have to find it in your app drawer and create a shortcut on your home screen (by long-pressing and dragging.)