You can't do that like this -- a scatter plot is only a single plot object so there can only be one legend entry.
I think to do so you'll need to draw each scatter plot by grouping variable where the grouping variable is the gear vector.
Alternatively, you'll have to create an additional handle for each of the six gears possible and map those to the same color map range. Plotting NaN values to create invisibile handles is a common trick to be able to do such enhancements that aren't directly available.
It would help and somebody might try to solve it if you'd attach a .mat file with the data;;;otherwise folks have to try to create something and we can't really see what you actually have.
