ArenaNet announced a Mac client for GW2 today.
It appears to be a Cider port, and while I'm downloading it at the moment, I haven't yet been able to check out its quality.
You can download it by logging in to your GW2 account, clicking "Download Client", and selecting the Mac client. This will download a disk image (.dmg) file with the installer, the installer will then download the rest of the client.