Gimbal means that the weapon is hung in a rotating construction, so it can move independent of the ship's orientation. Gimbal-mounted weapons can keep track of a target even when you're not pointed directly at the target, allowing far more firepower to hit a target. In exchange, gimbal weapons are usually weaker than their fixed counterparts and are vulnerable to chaff as it disrupts the weapon's ability to track the target. Also keep in mind that tracking systems track heat; if a target is not generating sufficient heat, gimbals will not lock on and behave much like fixed weapons.
Turrets are fully independent auto-firing weapons that have a 360 degree range of fire.