Whether you have a gas fireplace or a wood-burning fireplace, you are going to have a chimney. There may be gas fireplaces out there made to function without a chimney, but these are still far less common than ones that do use chimneys.
Your chimney plays an important role in keeping your home safe. This is because it is responsible for siphoning gas and smoke out of your home, rather than allowing it to fill the room the fireplace is in. Keeping your chimney in good condition is important for your safety and your comfort–and a chimney cap can help you with this.