That is rather odd as I have never heard of a phone coming without a Gallery app. Did you tap and hold the background of the home screen and see if there is an icon for the app in there? It just may not be on the home screen itself.
If you do find it there, then you can put the icon on any home screen you want and access the gallery that way. It could also be named Photos, etc.
Should you not find an icon in there, then I would go into the phone's "Settings" and then "Apps". Go to "All" and search for apps listed that are gallery or photo apps. Whatever name you find in there will be the name of the app you should see with all the icons (from the prior screen mentioned).
If by some wild chance you don't have anything at all, you can always get one from the google play store.