Forgotten Worlds can be played by up to two players simultaneously. The player controls a flying muscle-bound soldier armed with a rifle with unlimited ammo. The Player 1 character is equipped with a long-range automatic rifle, while Player 2 has a short-range wide shot. The controls in the original coin-op version consists of an eight-way joystick for moving the character in the air while flying and a unique rotatable button known as the "roll switch".Rotating the switch left or right allows the player to adjust their character's aim in one of sixteen directions, while pressing it causes the player character to shoot his gun. This allows for the player to move their character anywhere while keeping their aim in one direction. Pressing the switch rapidly will cause the character to perform a "megacrush" attack will destroy all on-screen enemies, but at the expense of a portion of their vitality gauge.
The player character is accompanied by a satellite module orbiting near him that will provide backup firepower every time the player fires their gun. Like the main character, the satellite can also be rotated with the roll switch. Rotating the character while firing will only rotate the aim of the satellite, while rotating the character without firing will not only rotate the satellite's aim, it will also move its relative position around the player.
The player can obtain blue-colored coins known as "zennies" from defeating enemies throughout the game. The zennies are used as currency to obtain new power-up items from shops located at certain points in each stage. When the player enters an item shop, they are given a choice of the items available and a limited time to make any purchase they wish. These items consists primarily of new weapons for the satellite module, but also includes a health kit to restore lost vitality, an armor that allows the player to sustain additional damage, and even tips on how to defeat the boss awaiting at the end of the current stage.
Forgotten Worlds consists of five stages with a total of eight bosses. The player will lose if their vitality gauge runs out, but will be given a chance to continue.
In Gain Ground, players control one of a set of characters at a time. There are twenty characters, each with different weapons. To beat a level, players must reach the exit point with at least one character or destroy all enemies on the level before time runs out.
Littered across the level are captive characters, which can be rescued by walking over, then escorting the controlled character to the exit point. If a player controlled character is killed, that character turns into a captive, except that they will disappear if the next active player controlled character dies, exits the level without them, or the player has no characters left in their party.
The game is over when all controlled characters in the party are killed without any reaching the exit. However, there are three continues which allow a player to restart the level with their original three characters.
There are 40 levels in the arcade version of the game. The Master System and the Genesis/Mega Drive have 50 levels in the game.