Gems will be an important item in the economy because they are purchased by players. I know that they can be traded for gold. That is a huge feature to the whole gems market that will be a gold makers dream. The most basic concept in economics is buy low, sell high and the potential for making tons of gold off of gems is no different. Because the entire market is driven on players purchasing gems for real money the price of gems will always be fluctuating. When a bunch of players buy them they will be worth less and when those gems are gone they will become more valuable. The way you make money on this is by anticipation and research. Unfortunately there is no way to know much about their pricing patterns at this point since we've got no legitimate market to reference at this juncture.
When the game launches there will be an incredible opportunity to make some serious gold. There will be no established pricing point so you could spend a few buck on some gems and sell them for gold. They will likely sell for much more in the first 48 hours than they will after that. If you are willing to spend $5-10 this may be the easiest way to get some capital and start your speedy journey to leveling up your professions.
Now if you aren't prone to spending real money on virtual item this is the other option. First--DO NOT buy any gems in the first 48 hours of the games launch for gold. They will be prices extremely high and players are going to be greedy trying to catch an early buck. Instead I would suggest you take note of the price as one of the first things you do when you log in on day 1. Then check that prices every few hours. If the prices drops then you keep waiting. Continue waiting for that price to fall. You will be amazed at how ridiculous the prices where when you logged in. This is common in every MMO to have ever been released. World of Warcraft, the game I came from, had this same thing happen with ores when Cataclysm launched.
Now that I've gotten the first 48 hours out of the way I will show you how to profit in the years after that. Or at least until they change things up entirely which isn't impossible.
The key to making money on gem sales will be in tracking the prices. There are common trends in MMO's in that prices rise an fall due to different levels of player activity. The game will have different levels of people playing it at different times. Day to day there will be more people online between the hours of 6PM and 10PM. Those are pretty standard peak hours for all online games. Then you have the weekly trends. More people will be playing the game on Friday and Saturday nights than any other time. These higher levels of player activity will result in more gems being purchased and thus the stock of gems will shrink. In turn, the prices of gems will rise due to that. Since gems are essentially a consumed item and they need to be purchased with real money its stock isn't as likely to be replenished as quickly as if it were crafting materials. At least that is general consensus across most of the community.
Going on a time slot of 6PM to 10PM is a great place to get started, but you will still want to personally track the prices because efficiency is key and you want to maximize the amount you are getting on your investments. You may be able to get 5-10% more just because you sold your gems at a certain time. It doesn't sound like much, but stretch that profit out over a few months and you will have accrued a nice chunk of gold. One of the major things I am covering in the Guild Wars 2 Gold Guide that I am working on is efficiency. There are always corners that can be cut for profit and it is important to identify them in order to squeeze extra gold out of your every interaction in the game.