Gordon isn't just some over-muscled oaf in a power-armor suit. He has a doctorate in particle physics. He doesn't need an AI to tell him what to do. He can outwit his opponents and manipulate the surroundings to overcome any obstacle. His survival skills in hostile environments are unmatched. Unlike MC, he doesn't have a technologically advanced army covering his back to assist in saving the world. Because he doesn't need one! All he has is a motley crew of outmatched resistance fighters against the entire might of the Combine. Gordon is the underdog.
Take away Master Chief's armor and he's a sitting duck. (His armor looks gay anyway.) And what kind of name is "Master Chief"? It's a slight step up from Master Chef. Hardly the type of name that strikes fear in heart of his enemies.