This guide is for those that don't know what GDKP is. The real long version can be found here: GDKP Run Discussion: Spread it to your Server - Elitist Jerks
I'll be making a TLDR version which... is also sort of long.
First, what is GDKP? It's Gold-DKP. Basically instead of spending DKP points earned in a guild through raiding and boss kills, you spend gold to buy a piece of loot that drops off bosses.
In short, players will bid on items that drop off the raid bosses. The increment raise can be as low as 100g to as high as 1000g. It's all up to your GDKP run really. At the end of the run, the gold is evenly distributed amongst the players. Depending on what items that drop and what they're priced at, you can walk away from that raid with quite a bit of gold.
For example:
Marrowgar drops the Bone Arbiter. The GDKP leader will then start the bid at the minimum price of, let's say 1000g with minimum increments of 500g. Players in the raid will bid until they have the highest bid. That gold is then traded to the GDKP leader, the item masterlooted, rinse and repeat. The gold is put into the GDKP pot and it is then split evenly amongst the the raid after the raid ends.
How does this benefit you? Well, it's a great alternative to people who don't want to spend the time to raid or simply have crap luck when it comes to rolls but too much gold in their pockets. This system, Gold is beyond anything else. If you're a cloth wearer and want a piece of tanking plate, you can have it if you spend more gold than the other guy.
Many servers already have GDKP but I was surprised to know not everyone knows about it.
For the lone casual player:
Getting into a GDKP is fairly easy. Simply type in trade channel during prime time, or check out your specific realm forums on the WoW official forums. This is a great way for you to get some ICC gear and with enough gold, some people will even let you buy the necessary materials to make a Shadowmourne. Just have a ton of gold and more often then not you'll get a spot in a GDKP.
For the well geared player (guild) that has a lot of well geared friends (guildies):
This is your lucky day to make a good chunk of quick and easy gold. If you have a guild that's currently on break til cataclysm and can't for a 25 man anymore but enough players to form a casual pug, this is a great way to make some gold. Just take you and around fifteen of your closest friends and the rest "buyer" pugs. The crappier their gear, the more inclined they will be to buy all the epics that no one else wants.
Obviously, the better skilled your hard carry team is (the well geared players), the more gold your raid is capable of making.