Gamebuino forum

Could we access to the SD card in order to read and write files?

For instance, to save and load txt files into a notepad app or music into a basic chiptunes compositor or to save images using a camera attached to an I2c port like the gameboy camera...

Yeah of course you can access the SD card to do whatever you want (read, write, create files and folders, renames them...). It will depend on the SD library you use. The more features it has, the more memory it will take.
You can find a list of the most common SD libraries for Arduino (...and Gamebuino as well) on the wiki : http://gamebuino.com/wiki/index.php?title=SD_card
Reference of the official Arduino SD library : http://arduino.cc/en/Reference/SD