In rare attic spaces, having just 2 gable vents will provide adequate ventilation. If one vent brings in cool air in the morning for the other to vent out the hot air and flip-flop in the evening. Gable vents are designed to freely allow air to pass through the wall area into or out of the attic to keep it cooler. But in most cases, soffit vents or cornice vents will better bring in cool air to force the hot air out. No matter what, there has to be 2 vented areas for this natural ventilation to work.
Having only 1 vent will slow the venting process and cause the hot air to sit too long and become stagnent. Stagnent or stale air is full of particles that produce an irritating smell to us. But to a home, it spreads mold and germ spores. These spores will float around in the air till they stick to a rafter or wall and begin to grow. Over heated attics are ideal for excess drying of rafters and causing dry rot as well as excess moisture growing mold.
Besides these issues, heating and cooling bills are much higher when the attic is not propoerly insulated and vented. By insulated, this does not have to be the roof or between the rafters, but the attic floor which is attached to the home's ceiling does. This insulated barrier protects from the heat loss of the living space below the attic. The home space alone is vented by windows and doors being opened. This brings in fresh air from outside, usually plenty. But the attic is not where anyone would be opening a window every day. This area needs to be as self sufficient as possible since most people never even enter their attic.
But the chance you might or even need work done in your attic, the summer can cause an attic to excede 160 degrees. This is the point that it can become combustable. Healthy attics rarely excede 130 degrees. The easiest way to tell if the attic needs more vents is to monitor the temperature on the hottest summer days. No matter what though, there has to be an intake and exhaust vent for any ventilation to happen. Or there is not enough air circulation to push the overheated air out fast enough to cool the attic.