If you don't train him, then he won't know what is expected of him (a trained dog is a happy dog
). If he knows the 'stay' command, it's pretty easy to teach what's off limits. Go into the room/on the stairs you DON'T want him on, then when he gets to the threshold, use the "stay" command. Reward immediately, then if he goes in say "NO", gently back him up and use the "stay" command again and reward when he stays. Take the time and be consistent and you'll have those gates down in no time
.