I cannot lock photo gallery from settings,
i would expect it to be in privacy under security in settings, but it doesn't show up there. either i haven't looked hard enough or you just cannot lock it.
Not all Android handsets have this sort of functionality. Most of the articles I've read mention a feature called, "content lock" for LG Android handsets.
You can download a third-party app that'll perform the same function. Try searching the Google Play store for, "Valuty" or "Keepsafe Vault."