Alien Gate is a fixed shooter. Each stage consists of the player's ship facing off against a giant head that spawns several smaller enemies. Once all the required enemies are defeated, the player must defeat the head as a boss. The background gives the impression of a vertically scrolling shooter, but the attackers stay in position relative to the player's ship. There are 25 stages and 14 different enemy types.

A terrifying evil is threatening the safety of your planet. Hordes of monsters are advancing through the Alien Gate with one objective in mind - the destruction of your world! Only you can ward off this danger. As a rocket fighter pilot, your clearheadedness will be tested to the utmost. The last battle has begun.