You can clean gold plated jewelry the same way you clean solid gold jewelry. The only thing to be careful of is not to scratch it. Put a couple drops of dish liquid in warm water. Do NOT use automatic dish liquid because it does contains abrasives. Let the jewelry soak for a few minutes to loosen the dirt. Brush the jewelry with a small, soft brush. I usually save my old toothbrushes for this purpose. You especially want to brush around stones and clasps to remove accumulated body oils and dirt. Then, rinse with water taking care not to let the piece/s escape down the drain (you can use a small wire strainer or mesh tea ball). Dry with paper towels or anything else that's lint free. Finally, wear your clean, shiny jewelry with pride!
A solution of half Mr. Clean and water with a small amount of ammonia can be used to clean gold-plated jewelry. This is what many jewelry stores use in their ultrasonic. Soak for 15 seconds then rinse with water and dry.