How to stop a dog from marking in the house?

My dog is so wonderful and totally house trained. He will not go in the kitchen but if he gets past the baby gate into the living room he will mark everything! When I come home he slinks outside with his tail between his legs and ears flat, before I even say anything. Like he knows what he did was totally wrong.... show more My dog is so wonderful and totally house trained. He will not go in the kitchen but if he gets past the baby gate into the living room he will mark everything! When I come home he slinks outside with his tail between his legs and ears flat, before I even say anything. Like he knows what he did was totally wrong. Any ideas? (He is a 4 year old German Shepard.)
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