GDKP raiding is becoming more and more popular every day, and with a good reason. It can be not only a good source of gold, but a way to gear your character in the meantime too. However, there are still lots of people who don't know about it. This post will definitely help them out.
GDKP raiding has many forms, and comes with various sets of rules. I'm trying to describe the most common one. A GDKP raid is held by a guild. Around 50-75% of the people are from the guild, the rest of the players are guests. The majority of the guildies assure completing the dungeon. At the start of the dungeon every player gives a small amount of gold (usually 1-2k) to the raid leader which (s)he will get back at the end of the raid. This is for filtering out the quitters. You're not likely to leave the raid if you have 1-2k in the pot.
Loots are "auctioned" for. Auctioning has about a million different forms, both in IRL and in the game, but usually it is an open bidding-war style auction. After the winner is determined, (s)he buys the loot for the amount (s)he won the auction for. This money goes to the raid leader as well.
This goes on for every loot of every boss. By the end of the raid, the raid leader will have a huge amount of gold (hundreds of thousands of golds), which (s)he splits evenly among the players (this assures that everyone gets back their 1-2k).
The amount of money the leader will have to split depends greatly on what loots drop, and how much money are the players are willing to spend on them. Usually, a GDKP raid, if you don't buy anything means at least 5k gold, the key word being "at least". I've heard of loots going for 60k in auction (not much after new content came out).
While the temptation might be great to start this right away, and never buy a loot to maximize the profit, getting invited back depends on spending money occasionally. It might be easier to get in if you are in the guild, or very close to the members, but in most cases GDKP raiding is a game of high stakes for the "high rollers". The profitability of the raid require the players spending too, and everybody knows it, so this won't be your magic fortune-maker, however it is a great opportunity to gear your character, and gain some golds too, if you are auctioning cleaverly (or you have lower competition then others on your loots, for example by being the only rouge in your guild).