The Raspberry pi has 26 pins (not all of which are IO), and the geekport has 37 (again, not all of which are IO, but still has more IO ports than the Pi). This means that a full converter would require some intelligent circuitry to multiplex the PI's GPIOs. The bigger problem is that the geek port has some 12V pins, and analog pins, neither of which the Pi supports. Again, this can be solved with some circuitry in the converter.

Using this converter would require the software running on the PI to be aware the intelligent circutry and either have driver support for it, or operate at a low enough throughput to bitbang in userspace.

Could use the Beaglebone Black, it's has more than enough GPIO pins and has dedicated coprocessors for bitbanging, which improves throughput and latency.

http://kilobaser.com/blog/2014-07-15-beaglebone-black-gpios

http://hackaday.com/2014/06/22/an-introduction-to-the-beagle...

My favourite use of the Beaglebone Black so far is as the core of Bela:

https://www.kickstarter.com/projects/423153472/bela-an-embed...

Think with the BBB you have to be super careful with those GPIO pins (more so than the RPi apparently) - have heard of people having fried their Beagle as a result of a tiny mistake.

The GPIO pins on the BBB are 3.3V tolerant, but not 5V tolerant like the Arduino. RPis GPIO has the same power constraints as the BBB (3.3V tolerant, but not 5V tolerant).

It's fairly straightforward to check if you're going to need a level shifter, takes seconds to check voltage with a multimeter.

Only the first two RPi models (A and B) have 26 pins. The later ones (A+, B+, 2B, Zero, 3B) have 40 pins.