oops, not Gettysburg, I thought you meant Battle Cry.
Vassel- you play against a live player (either directly or email), you both need versions of the game you are playing, Vassal just displays the game to both of you.
on the front page scroll down down to 'WHO'S PLAYING', click the 'server status' then click on 'MONTH, that will show what games were played in the last month, Gettysburg isn't that popular yet. check the forums for lots more info. I used to use Vassal lots (years ago to play Bloodbowl), and I'm just about to start using it again C&C Ancients.