I think what would be much more interesting and useful than a Gertboard is to use something like
Leonardo or Teensy along with something like Firmata so that the RPi can do all the high level work
and external communications and then let the Arduino handle all the real time i/o like PWM, i2c, servos,
A-D etc... as well as the 5v interface.
So then it becomes as simple as plugging in the Leonardo or Teensy into the USB port on the RPi.
This would make it much simpler/friendlier (and cheaper) for beginners.
Teensy would offer a great interface for breadboard experimenting
and Leonardo would offer access to using many of the existing Arduino shields.
It allows the RPi to be sealed up while the IO handled externally by the Arduino hardware
over USB.
--- bill