I am not an expert on the theory of attic venting, but from what I've read about attic venting, it serves 2 main purposes. 1. Removes moisture from the attic. 2. Prolongs the life of your roof.
Another factor in cold climates is ice dams.
The ideal sytem is a ridge vent with continuous soffit vent. This forms a chimney effect. Air enters via the soffit vent. Travels along the underside of the sheathing (in the channels formed between each roof rafter), bathing the sheathing in a continous supply of outside air. Then exits via the ridge vent. In the process, not only is the moisture removed, but the roof is cooled in the hot summer. Also very importantly, in the winter the underside of the sheathing is kept at near the same temperture as outside air, so that snow is less likely to melt during the daytime and refreeze at night causing ice dams. Also attic moisture is sweep away and doesn't form ice crystals upon contacting the uderside of the sheating. If you don't have continous soffit vent, you should consider installing them. As you can see the gable vents play little part in this type of venting. However, I would not close the gable vents, especially if you don't have sofit vent. If you have soffit vents I would only close the gable vents if you live in an area subject to high winds that are causing a problem by blow attic insulation around, etc.
If you search the web for "ridge vent", I think you'll come up with more good info.
My lack of venting knowledge, resulted in me blocking my soffit vents while blowing extra insulation into my catheral-ceiling ranch home here in Ohio. The attic is not accessable because the roof sheathing is on one side of the 2x8 rafters and the ceiling is on the other side.
The first spring after turning on the air conditioning, I thought I had a roof leak, because water start dripping from the base of the hall lighting fixture. When this continued even when we hadn't had rain for over a week, I realized that I had moisture build-up in the insulation. Since there had never been a ridge vent, I installed one, hoping that it would solve the problem. It helped some. After a few more years, I notice that my roof felt spongy when I walked on it. One neighbor told me of someone living in our developement that had their ceiling fall in as a result of a moisture problem after adding insulation.
I had to have my roof completely torn off, insultion removed, soffit vents cleared of insulation, dams installed to prevent the remaining insultion from migrating back over the sofit vents, new sheathing, and shingles.
It enough to make a guy get interested in venting!