GW is f2p, but it's one of those games you have to buy first then its free for life... There was a huge sale of the game on steam, and the game is still pretty cheap I believe...
If you're not sure just buy one of the games, then if you like it, just buy the rest. My suggestion though is to get Nightfall first, since it gives you access to heroes which are fully customizable and way better NPCs you can party with if you decide to NPC solo. Prophecies is good, but you're limited to henchmen, which are like heroes, but weaker.
You might be able to get it from the NCSoft Launcher from the site, too...