Alien Gate is a vertical scrolling shoot 'em up where you fly a spacecraft over alien terrain while shooting down enemies. The game features 25 levels and comes with a variety of different enemies, including birds, bees, bombs, centipedes, UFOs and knives. As the game progresses, the giant head that spawns these enemies becomes increasingly aggressive.
The gameplay involves navigating your spacecraft through each level while avoiding enemy fire and trying to destroy as many enemies as possible. The ultimate goal is to reach the giant head-boss, which will only appear at the end of each level. Defeating this boss will give you a brief respite before the next level begins.