Gears are versioned by Semver Standards.
1.2.3 = Flywheel gear version
4.5.6 = Author's Algorithm version
A site can have different versions of Gears running at the same time.
The default version option will be the Gear most recently uploaded to the site.
You can select a Gear to run with a specific version configuration. Note, that if an older version is preferred you'll need to disable the "Always use the latest gear version" toggle.