I've always been a big Sega fan. In my personal videogame developer Canon, Yu Suzuki is the undisputed God of the Arcade. That said, Yu Suzuki didn't actually have anything to do with Gain Ground...aside from being Sega related.
Gain Ground was originally released in the arcades back in 1988 (and later ported to the Master System, Genesis, and Turbo-Graphix 16, and later still remade on the PS2 as part of the SegaAges collection). It was rather unique in that it supported 3 players (not 2, not 4; 3), although the console ports only supported 2 players (obviously, the game would work well on Xbox Live regardless). Most multiplayer games back then were either straight-forward beat 'em ups or shoot 'em ups. Gain Ground has more in common with Gauntlet than it does with Double Dragon or Gradius. However, unlike Gauntlet, each player had multiple warriors to choose from. Starting at a mere 3, players were encouraged to pick up straggling warriors across its many battlefields. Also unlike Gauntlet, Gain Ground featured static levels; that is, they didn't scroll. What you see is what you get in Gain Ground, which cuts out the hack and slash monotony of Gauntlet and replaces it with a more action-packed arcadey feel.
Instead of trying to find a secret exit, or search of treasure, Gain Ground simply has the player trying to accomplish one of two goals; kill everything or have all players cross the Exit barrier. Obviously, killing everything is always the best option, but sometimes the player might find themselves in a situation where that isn't possible. Another thing that sets Gain Ground apart from Gauntlet is it's use of two ever-present attack options. Typically, this involves a speedy short-ranged attack, or a stalled long-ranged attack. The short-ranged attack works best on enemies that are "up in your grill," which is more often than not the case. However, aside from being useful with non-moving enemies, the long-ranged attacks USUALLY are able to bypass foreground barriers, i.e. they can hit enemies that might be on an inaccessible hill or rooftop. This is why sometimes it isn't feasible, or even possible, to kill every enemy on screen. The player might be stuck with the "ground specialists," characters that didn't have a long-range attack that could pass through foreground barriers; their only choice would be to avoid those enemies and have their crew pass threw the Exit barrier one at a time, extending the time spent on the level, and therefore decreasing their overall time-bonus.
From a purely story based perspective, Gain Ground wasn't super unique. You and a group of warriors are trapped inside an evil computer simulation, and must rescue your fellow soldiers. However, what makes the setting unique...or rather, convenient, is that the creators of Gain Ground weren't limited to a strict setting for their game. Wanna fight cave men? Let's do it! Wanna fight giant bugs in a scary desert? Sure! Wanna fight robots? Why do you even ask!? Recruitable characters are also diverse; from tribal cave-men to futuristic super soldiers. One would think that the future characters would always be a first choice, but many players found that the "bow and arrow" characters were the most well-rounded, and therefore almost always the best choice.
It is worth mentioning that Gain Ground has a really rockin' soundtrack. Not only did each 'Age' have different music; each piece of music was super lengthy! Most songs didn't loop until a good 2 or 3 minutes, which for the time was pretty impressive. It helps that each song is also SUPER catchy, and tends to get stuck in your head.
Enough of this explaining why Gain Ground is so awesome. Here's a list of potential achievements:
Gained Ground: Beat the game.
My Favorite Character: Beat the game on Extra Mode with only 1 character.
None Left Alive: Beat the game having killed every single enemy.
Conscientious Objector: Beat the game having killed only what's necessary i.e. bosses
Take No Prisoners: Beat the game on Easy mode having never rescued any survivors.
My Own Clone: Rescue a survivor with the same character.
Cyber Funk: Using Cyber, kill five enemies with a single shot.
Your Name is Mud: Using Mud Puppy, kill all enemies on screen without moving vertically.
Trapped in the Matrix: Using Professor or Kid, kill 2 enemies with a single shot.
Not Fair: Using Water Knight, kill an enemy after trapping it in a water spout.
Pinball: Using Zaemon, kill 7 enemies with a single tornado.
General Disarray: Using General, kill every enemy on screen without ever disengaging his flamethrower.
I love Gain Ground, and you should too.