Using Gertboard as an Arduino Interface
You want to program the ATmega chip on the Gertboard as if it were an Arduino.
Follow this detailed tutorial to set up the Arduino IDE, the Raspberry Pi, and the Gertboard to program the Gertboard’s processor as if it were an Arduino.
Discussion
The Gertboard ATmega processor operates at 3.3V, removing the need for any level conversion.
See Also
For a basic introduction to the Gertboard, see Recipe 8.17.
The Gertboard manual is available from multiple places.