Spray 3 1/2- to 4-quart slow cooker with cooking spray. Place chicken thighs in slow cooker, layering if necessary. Top with 1/2 cup of the dressing, and sprinkle with garlic; refrigerate remaining dressing for later. Cover and cook on Low heat setting 4 to 4 1/2 hours or until juice of chicken is clear when thickest part is cut (at least 165°F).
Remove chicken to rimmed serving dish, and discard cooking liquid. To serve, drizzle remaining 3/4 cup dressing over chicken; top with tomatoes and basil.