The Starred Alien version of Gory has much higher health and damage, along with more knockbacks and slightly increased range, it also has a barrier with a breaking point of 6,000 damage.
General GreGory can make a dent in your defenses, especially if they appear in groups. The barrier prevents General GreGory from being knocked back by the initial hit if the cat unit doesn't have Barrier Breaker (such as Fencer Cat), meaning that GreGory can keep pushing towards your base. Make sure you can hit GreGory again after destroying its barrier to kill it. This may be problematic to do in stages with deploy restrictions, as you may not be able to deploy a cat to kill GreGory until later.