Garth Brooks is set to headline the charitable Mack, Jack & McConaughey Gala next year. The annual Mack, Jack & McConaughey gala will place at ACL Live April 27-28 in Austin, Texas. The Mack, Jack & McConaughey gala is a two-day fundraising event hosted by former Texas Longhorns football coach Mack Brown, musician Jack Ingram…