I thought the question was about using gearing in general.If you are building an alternator then gearing will reduce your output compared with having the alternator right in the first place. If you want lower cut in speed then wind it with more turns.

If you have an existing alternator that is too fast then you can gear it but the result will never be as good as a more suitable alternator driven directly.

Gearing hurts you most in low winds so if it is the low wind area you are trying to improve then it's not a good idea for best results.

The only sensible reason to use gearing is to reduce alternator cost and this may become an issue with large machines. It is possibly cost effective for machines above 12ft compared with using more magnets and copper, but in the long term nearly all forms of gearing reduce reliability and now that we have fairly cheap neo magnets I wouldn't consider any form of gearing below 20ft.

Before the days of neo there was a far better case for it and above 8ft it worked out cheaper, but in those days very low wind performance just wasn't viable so a bit of gearing loss was often offset with less iron loss.