The short answer to this question is:

No. A gold guide is not necessary to earn lots of World of Warcraft gold.

Of course there is a long answer. There's always a long answer isn't there?

And this longer, and more involved, answer is that the need for a gold guide depends on a number of factors.

1. How long you have been playing World of Warcraft. If you have been playing since launch, you probably know a lot about the game. You'd know where to go to grind for gold. You'd know the best monsters to kill. You'd know which items sell best on the Auction House, and how to play the Auction House to make a killing.
2. You are a hardcore player. If you are a hardcore player, you would be in the game a lot. There's a really good chance that you have already gone through the pain of farming gold in order to afford that Epic Flying Mount. You've probably even bought one or more gold and leveling guides to help you on the way.
3. You spend lots of time outside the game reading blogs, forums, and game wikis. This alone will likely make you more knowledgeable than the average player who just plays the game. By extension, you are then likely to know more about farming WoW gold than the average player.

So. If you're a hardcore player who has been playing World of Warcraft for a substantial amount of time, and you do your homework by reading everything there is to know about the game, then you probably don't need the assistance of a gold guide. In fact, you probably already own several guides anyway.

Okay. Maybe I'm being a bit extreme here. You don't need to be a guru to make enough WoW gold to have a comfortable living in World of Warcraft. You can probably get by with picking one or two gold making methods and practicing those until you become an expert on them. I'd say you probably could even do that after being in the game for a few months. It really is just a matter of working out what works and then doing it.

Of course the people who write these WoW gold guides have already figured out all the details. They've scoured through every nook and cranny of World of Warcraft working out the best, most effective ways of making WoW gold and writing this information down into detailed guides. It may not really be necessary to use a gold guide to make lots of in-game cash, but having the insider knowledge sure makes life easier.

One of the better WoW Gold Guides, with tons of useful info and tips, is Warcraft Millionaire.