If you use a magnetic reed switch, there's some more things to consider.

First you'll need to determine if the gear warning sounds when the switch is open, or closed. Both types of switch are available, with magnet present a switch may be closed (the most common type of reed switch) or it may be open with the magnet present (there being a tiny biasing magnet within the switch that closes the reed when the external magnet is moved away.

Then there are "wide gap" reed switches too. These are magnet polarity sensitive, so the external magnet needs to be positioned so the correct pole approaches the switch (these switches will still operate if the external magnet is positioned incorrectly - they will just revert to a very small gap).

If the magnet is to be mounted on steel (as in tubing etc.) then better performance will be had if the magnet is not mounted directly to the steel.