For a true gable vent it does need to be vented. Gable vents are vented to allow fresh cool air to enter the attic and hot stagnet air to escape. If an attic or loft is vented properly by other means, it can have faux or non functional vents as decorative vents or vent covers. Proper ventillation of any loft area is determined by the attic temperature. This should never be allowed to go above 130 degres.
At 130 degrees, the attic becomes an oven. As any oven, high temperatures will cook the rafters and start dry rot. This doesn't stop there. It will overheat the roof decking and make the roof undrlayment brittle. Later it can actually ovr dry the shingles depreciating their life by over half. Improper ventillation is much more costly than installing the proper gable vent.
Attics reaching 160 degrees, even for a short time, can become combustable. This makes improper ventillation very dangerous for some attics. Paper and splinters are usually the first to cause a fire. But also, overheated wiring can be a dangerous culprit.
To know if your attic or gable wall vent needs to be vented, simply insert a thermometer at the hottest part of a summer day. This will tell you better if it is properly vented or not. Since attics and lofts are different shapes and usually blocked when people use them for storage, there is unfortunately no true formula to say exactly how much ventilation is needed.
By adding cornice or soffit vents to allow cool air in, the gable vents will function much greater. As heated air rises, the cooler air assists in pushing it out.