Due to the shape of a half moon gable vent they cannot be operational in the sense of opening and closing.
Your best option would to be to install a roof mounted vent capable of handliing the amount of cubic feet per minute (cfm's) of air flow which the fan produces. This roof vent can be installed easily but since I do not know what material your roof is constructed from, would recommend you contact a qualified roofing company on line.
If you would like to do a bit of research on your own, visit Greenheck's website here. They are a custom commercial vent manufacture and can do just about anything....for a price.
For more information:
Read "How do you determine what size fan to put in an attic to cool it properly?" a Q&A answered by Joshua Lloyd.