Season the chicken breasts evenly with salt, black pepper, garlic powder, onion powder, dried thyme, and dried rosemary.
Place the seasoned chicken breasts in the slow cooker in a single layer.
Cover and cook on low for 4 hours until the chicken is cooked through and tender.
About 30 minutes before the chicken is done, melt the butter in a medium saucepan over medium heat.
Whisk in the flour and cook for 1-2 minutes, stirring constantly, until the mixture is golden and bubbly to make a roux.
Gradually whisk in the chicken broth and continue whisking until the mixture thickens and becomes smooth, about 3-4 minutes.
Stir in the milk and cook for another 2 minutes until the gravy is creamy and heated through.
Remove the cooked chicken breasts from the slow cooker and place them on a serving platter.
Pour the prepared gravy over the chicken breasts or serve it on the side.
Serve immediately with your favorite sides such as mashed potatoes or steamed vegetables.