Another member of the list who adopted the GK rules to the Babylon 5 setting needed a scenario for a game he was running. He gave me the basic situation, and here's what I came up with:
FORCES: The Narn player gets several small military vessels (DDs and DLs) escorting eight to ten civilian ships--one escort for every two or three civilians. For the civilians, just take a cruiser or larger stat sheet and replace everything but sensors, engines, and jump drive (and a few light particle beam batteries if you want the Narn civies to have a little protection) with cargo holds.
The Centauri player gets three cruisers.
The human player gets one battlecruiser.
SETUP: Narn ships set up on the south half of the map, heading north with a drift of 4. Centauri ships set up on the west edge of the north half of the map, heading southeast with a drift of 5. The human ship sets up on the east edge of the northern half of the map, heading southwest with a drift of 3.
SPECIAL RULES: The human player cannot enter the battle until at least one civilian ship is destroyed. After the combat phase, the human player must make a successful aggression roll to join the battle by rolling a number on a d10 equal to or less than the number of Narn civilian ships destroyed so far; this represents the captain of the human ship having enough intervening to prevent the slaughter of any more innocents. The human ship may subtract one from the total of an aggression roll in exchange for two points of damage to a randomly rolled location; subtract two from the roll in exchange for four points of damage to a randomly rolled location; or subtract three from the roll in exchange for six points of damage. This represents the captain using the human vessel to shield civilian ships from Centauri fire, and the human ship takes the damage whether or not it makes its aggression roll. Until it joins the battle by making this roll, the human ship may not attack the Centauri and they may not attack the human ship.
VICTORY CONDITIONS: The Narn player gets one point for every civilian ship that leaves the map edge. The human player gets one point for every civilian ship that leaves the map edge after the human ship joins the battle. The Centauri player gets two points for every Narn military ship destroyed, one point for every Narn civilian ship destroyed, and five points for destroying the human ship if it has joined the battle. The player with the most points is the victor.
Of course, this scenario is easily adoptable to the standard Galactic Knights setting (or the setting of any other spaceship combat game, for that matter)--just substitute the force of your choice for each race.