Hey there,
To start, I thinks Sparkfun did a good
overall presentation of I2C.
Then, to know more about how to use I2C in the case of Arduino/Gamebuino you might want to take a look at the
Wire Library reference and examples (it's the I2C library packed in Arduino). There is also PongMulti in the Gamebuino examples which shows how to make 2 Gamebuino communicate both ways in real time over I2C and detect disconnection on both master and slave.
About the Gamebuino accessible pins, there is
the ones from the I2C port, A4 and A5. You can either use them to communicate with several I2C devices (or other Gamebuinos) or as raw pins. But not both at the same time. Note that there is pull-up resistors of 4k7 on these pins, but you can disable them by cutting the jumper (on the back side of the PCB, next to the left I2C port).
You can also access the
pins 11, 12 and 13 through the ICSP port (respectively MOSI, MISO and SCK). But be careful, they are used to communicate with the screen and the micro SD card (using SPI).
I hope that helps