This package includes two sets of gears so that you can modify your GM17 to a 51:1 or 104:1 ratio.
As it is, the GM17 has a ratio of 228:1 which makes it great for all sorts of purposes, but a mobile robot platform with 4-5 cm wheels needs a gear ratio of 50-100. Didel has a solution, offering this package which contains two sets of two gears that can convert your GM17 to a 51:1 or 104:1 ratio.
The installation is pretty straightforward - all you have to do is open up you gearmotor by removing the two Phillips screws, replace the lower two gears with the ratio set of your choice, and put it back together. Comes with basic instructions.