GDN mostly consists of a simulator, which is a room with some bumpy terrain. Stuff flies out of the walls, ceiling, and floor, and you shoot it to score points. It has a well-done physics engine: balls bounce around, off each other, or off the ground, and the projectile weapons are affected by gravity. The stuff is all abstract. There are colored balls, squares, and pill-shapes, as well as glowy-sparkly things that fly around. And others. And the music is great. GDN uses music by some of the best MOD composers online, so it's all good stuff. The "stages" are basically the songs, each of which is a custom-built level with stuff flying out in time to the music. There are high scores for each song. If you do well, you'll get "credits", which you can use to upgrade or buy stuff for your weapon.
There are a couple of different weapons to pick from, each with different properties. The lightning gun is the default; it shoots a lightning streak at the point you clicked. There's also a missile launcher with seeking missiles, a steel ball railgun, a machine gun, and an explosive shell cannon. The lightning gun is probably the easiest to use because it's pretty much instaneous. All of them improve when upgraded a couple of times, which usually increases the fire rate and/or power of the weapon. There are also some additions that do bonus things, like an auto-aim laser that will eliminate one target when you right-click. (Upgrade it for more uses.)
The other main part of the game is the "War Room". Basically, there are these meteorites with infestations, and you have to go eliminate them. The War Room shows a map with locations on it, and you can travel between them (which takes time, and you only have a limited amount). You have to clear out infested locations, which are played in a room like the simulator's, but with different objects. In the War Room, objects actually hit you if you don't destroy them, reducing your shields. If your shields are destroyed, you'll have to retreat to the nearest safe place. The object is to reach the location where the meteorite is located, and destroy it. It's very much a race against time to find and destroy the meteorite. Succeeding at missions advances the plot, kind of. There's not much plot. Something about evil alien meteorites.
I really liked GDN, which surprised me. I'm generally not much into shooters, but GDN is really polished and, well, shiny. And I really like the music. It's also nice to sit down and play in short bursts, since you can just play one song at a time. (Yeah right I play "just one song" at a time...) As far as gameplay goes, it's pretty solid. I don't have much experience with other games in this genre, so I can't say that much. But it's good. :P
My CLoA: 17,251,703 on New York Lights. Max multiplier 126.42x for a total of 17,083,052 points with a longest link count of 160. Accuracy 98%, cleared 63% (10/15 clean sections) And gar those little glowy dots which are great for keeping your link up but not so great for percent cleared.... Aptitude was only 197.3 on this one.
(old: 10,612,010 on Virtual Wonders. Max multiplier 90.06x for a total of 9,654,527 points. Accuracy 90%, cleared 97% (10/13 clean sections). Reaction speed 147.1, aggression 6.7. Aptitude 404.7)
(older: 7,744,327 on Happy Dayz. Max bonus 85.69x for a total of 6,615,590 points. Accuracy 94%, cleared 78% (6/17 clean sections). Reaction speed 65, aggressiveness 8.5.)
Come on, Will, I know you've been playing... :P
My CLoA: 8,804,769 on Happy Dayz. Max bonus 69.2x for a total of 5,611,636 points. Accuracy 88%, cleared 81%. Reaction speed 72.6, aggressiveness 8.5.