The GPIO schematics have always been in Supplementary Downloads on the official website:
http://blog.petrockblock.com/supplementary-downloads/retropie-gpio-adapter-schematics/
A parts list and explanation of the circuit is on the blog post announcing the adapter:
http://blog.petrockblock.com/2012/10/21/the-retropie-gpio-adapter/
Here is a higher quality image of the board:
http://blog.petrockblock.com/wp-content/uploads/2012/10/P1010401_small.jpg
AIUI the GPIO adapter is out of stock because it's been superseded by the ControlBlock: