Place chicken gizzards in a large saucepan and pour in enough water to cover. Simmer over low heat until tender, approximately 30 minutes.
While gizzards are simmering, mix together onion, bell pepper, garlic, vinegar, oil, bay leaves, olives, and peppercorns in a large bowl. When ready, drain gizzards and mix into bowl. Season with salt to taste and marinate at least an hour in refrigerator.