Gain Ground, although ok with a single player, demonstrates its brilliance if you can convince a friend to tag along.
Essentially, you win by gaining ground. You have to traverse a map and either get your characters to the exit or kill all of the enemies within the time limit. It's a simple premise but the game has a surprising amount of strategic variation. Knowing which characters are suited to a given situation is crucial to victory. Even then, the game poses a stern challenge due to the sheer amount of bullets, arrows, lasers, missiles, knives, etc. thrown the player's way. It's a brutal, unforgiving game. Especially the Gain Ground AI. The end boss in particular is unforgiving - homing missiles which you can dodge? Argh! And all for a pretty horrible ending.
Personally, I couldn't disagree more with Gamespot's VC review of 'Gain Ground'. Although the game doesn't explain how to play properly, that doesn't make it a bad game. I see that as part and parcel of retro gaming and although I understand they're reviewing for a modern audience I find the criticisms unduly harsh and anachronistic. In whichever format I recommend it for the multiplayer experience.