Melt butter in a skillet over medium‐high heat. Add onion, bell pepper, and celery; sauté until softened, then add garlic and cook until fragrant.
Stir in sliced sausage and sauté briefly to warm through, then remove from heat.
In the slow cooker, whisk together the water or broth and Creole gumbo base mix until combined.
Add the sautéed vegetables, sausage, chicken, and fire‑roasted tomatoes to the slow cooker. Stir to combine.
Cover and cook on high for about 3 hours, or on low for 6–7 hours, until the chicken is tender and the flavors are well developed.
Remove the chicken and shred it with two forks, then return it to the slow cooker.
Stir in the okra and shrimp, cover again, and cook for an additional 30 minutes until the shrimp is cooked through and pink.
Taste and season with salt and pepper as needed.