How do I reverse my electric motor?
The components on the back of the motor are three capacitors to suppress the contacts, and a diode to protect whatever switched it from a back-emf. 

If you reverse the polarity without removing the diode, then you will just put the current through the diode not the motor, and possibly either destroy it or damage your power supply, or both.

So you need to remove the diode, and if you are using a simple transistor to switch the motor, provide some other protection. If you're using an h-bridge with diodes, then you're ok (I'd imaging the gert-board does, but don't know for certain).

You also need to remove the larger black capacitor - it is polarised, so will be damaged and possibly explode if you connect it the wrong way round. Previously the diode prevented the current going through it when you did that. If the motor generates lots of electrical noise when you run it without the capacitor - interfering with radios and so on - then you will need to add a similar value of non-polarised capacitor to suppress that.

Also check that board you're using can supply the motor's required current.