Pak, the only way to know for sure is get in the attic with one of those smoke pencil things and analyze what's happening. The "problem" with the gable vents is the air may not be flowing up from the soffets. This can overheat shingles and induce heat-related failure. I wouldn't think that would be a problem in your area. Worst case scenario would be to block of the gable(s) for the summer and open it up in the winter.
