Just received a GSC-10 yesterday and will be installing tonight. Can I use the magnet from my current speed/cadence device (Cateye double wireless), and just use the main unit from the Garmin? Seems silly to take a couple magnets off only to replace them with what may be the same thing.
Yes. A magnet is a magnet and as long as the magnet's field passes close enough to the sensor for it to register, it'll work just fine. The only way it wouldn't work is if it was a magnet with a weaker field, but then you'd just have to adjust it so that the sensor sat closer to it.