They dropped the old 'gallary' app when they went to 5.0. Google Photos is the default picture app on a Nexus now. You can access your local files by going to the slide out menu and selecting 'Device Folders'.
By the way... use Google Photos' sync feature. They offer unlimited storage (the unlimited option compresses the pictures, but there is no real loss in quality... really, people have analyzed the before/after with graphics tools and the compression is extremely good).
Or you can download an app like Focus from the Play Store. That's a more traditional gallery app.