With tip of knife, make cuts in top of beef roast; insert garlic slivers. Spread beef with 1 tablespoon of the horseradish. Sprinkle with 1/2 teaspoon of the gravy mix. Place beef in 3 1/2 or 4-quart slow cooker. Arrange carrots around beef. Top with potatoes and celery.
In small bowl, combine 1/2 cup water, remaining gravy mix, salt and pepper; mix until well blended. Pour over vegetables.
Cover; cook on Low setting for 8 to 9 hours.
Just before serving, remove beef and vegetables from slow cooker; place on serving platter and cover to keep warm. In medium saucepan, combine cornstarch, 3 tablespoons water and remaining 2 tablespoons horseradish; blend well. Pour juices from slow cooker into cornstarch mixture; mix well. Bring to a boil over medium-high heat, stirring constantly. Cut beef into slices. Serve beef with vegetables and gravy.