When Eggman blows into the HQ on Prison Island, the security systems try to block him off by closing all metal doors, which must be destroyed to pass trough. The goal is to destroy the G.U.N. robots and penetrate the entire base. Throughout the level the security system will alert enemies to Eggman's presence by announcing the level you are on. The security of the level gets tougher depending of the security level Dr. Eggman is.
The layout of Iron Gate is basically the same process with new twists when you enter a new room. Most of the time the player must try and shoot locks off a door, or try and destroy enemies while on a moving platform. Iron Gate is just a level to introduce to the player what future levels are like.
Eggman's Laser Blaster is found here.