Community rep Bayohne confirmed today that because early phases of beta are still in Gridania (like the alpha), normally that would restrict players to selecting a class with its guild headquartered in Gridania (Archer, Lancer, Conjurer).
However, due to content requiring a proper tank which will be available in the first two phases, such as the level 25 dungeon, players will be able to unlock gladiator.
I had been under the impression that all classes would be available before the third stage of the alpha, but given that a good chunk of the early content in the game is class-specific content (which in Alpha was a class storyline quest every five levels, which provided class-specific items, usually new weapons), if we're still limited to Gridania until the third phase of the beta when we can import our 1.x characters, it would make sense that we don't get access to the other jobs.Originally Posted by Bayohne
Still, it's good to hear that there's going to be plenty of content requiring a full-party set-up. Between gladiator, lancer, archer, and conjurer, you can put together a well-rounded team. Also good to hear confirmation that the level 25 dungeon would be included (Alpha only featured Tam-Tara Deepcroft, which was level 15; I never did a run while I was messing around in Alpha, but I heard good things about it).
Source: FFXIV Official Forums