Gravitational Turrets are an advanced ship type, and are not meant to be unlocked at the start. There are a number of turret types (laser, missile) that follow this same pattern, and most likely more will come in the future (with all ship types -- such as Dreadnoughts in the Starships section). It's simply a matter of the value of the ship class, whether you get some of it for free always, or if you have to unlock even the first level. This is pretty consistent with a number of other RTS titles I've played, actually.
