Welcome to Android Central! Google Photos will show you the photos that are saved locally on the phone, as well as any photos that have been backed up to your Google Photos in the cloud. The app itself doesn't contain photos, so if you uninstall or disable it, that won't get rid of the photos -- a person could still use other gallery apps (or a file manager app) to see the photos that are stored on the phone. Google Photos is often preinstalled, so you usually can't uninstall it completely.
Who else has access to your phone? Why not just have a screen lock with a strong password? Unless it's a parent or guardian, no one has a right to invade your privacy.