When you connect to the server, you enter the lobby. You'll see a list of everybody there. You'll need an opponent to play, so if you're the only one there, get a friend on, or log in again in another window with a different name to play against yourself. Names in green are players who want to start a game. You can click on a green name to start a game with that person, or click the Host button and wait for somebody else to click on your name.
Gravity Wars is a two-player, turn-based game. The object is to hit the other player's ship with a missile before he hits yours. You get one shot per turn. To fire a missile, enter its direction (0-360, with 0 being to the right and 90 straight up) and speed (0-10), and click the Fire button. You have to use the gravity of the planets to guide your missile to your opponent's ship. The planets are fixed in place, so on each turn you tweak your firing direction and/or speed until you score a hit. Most of your tweaking will be to the right of the decimal point.
Red planets are the densest, followed by green, and finally blue. A denser planet exerts a stronger gravitational pull for its size.
You can modify the style of the game with the options menu. Pick 'Many Planets' and start a new game for a greater challenge. You can also change where your ships appear. By default, the red player is on the left side and the blue player is on the right. Select 'Ships Anywhere' and start a new game to have your ships appear anywhere on the board.
(The options menu is a bit confusing. When you pull down the menu, the options displayed there represent the options you would be switching to if you selected them, not the currently selected options. Sorry about that. I wrote this game back in 1998, when I wrote user interfaces as an afterthought.)
Can't connect to the server? You are probably behind a firewall. The Gravity Wars server listens for connections on port 5003. If you are behind a firewall, then it must allow traffic through port 5003 for you to be able to reach the server.
Please report server problems to firstname.lastname@example.org