What should i eat besides protein to gain and maintain muscle weight?

Avoid the protein: Protein doesn't lead to gain in muscle mass/weight. It's broken down in the stomach and metabolized. Otherwise, you'd end up looking like what you eat. A high protein diet can damage the kidneys. Eat a healthy, well-balanced diet and exercise vigorously and you'll gain muscle weight.

Healthy carbs.: If you're working out, you need protein to rebuild muscles and promote growth after the workout, but you need healthy carbohydrates for energy to power you through those muscle-building workouts. Carbs are stored in the muscles as energy stores called glycogen.