You can't. This isn't supported; you can't be an op but be unable to access these commands as /give and /gamemode belong to Level 1 (cheats) permissions; second-lowest and will lock them out of other commands.
See What does op-permission-level do?.
Permissions Way (Requires Plugins and Bukkit/Spigot/Paper)
Give the following permission nodes and ...
Yes! As long as the server is properly configured, skins should be visible to everyone, custom or official.
Steve and Alex are the only fully official skins.
If you want to download a skin, NameMC can be used. Simply type in a username.