Question: I want to connect a PCIe 16x 3.0 GPU card to a Raspberry pi. I don't mind lo loose bandwidth, i just want to use the core GPU capabilities. So, the only idea that came out to my mind is to use the 26 GPIO RPi pins. I am concern about the GPIO/PCIe programing, do not have any idea neither an approach. Is this a crazy a idea ? can somebody help me to get a big picture of what i must to do ?

Answer: 
Your best bet would probably be to check whether a [PLX PCIe bridge](http://www.plxtech.com/products/expresslane/bridges) would be usable for this purpose.

It's highly likely that abandoning this idea of using a Raspberry Pi and instead using something that has a PCIe port built in instead will be vastly easier and cheaper.
A few sites have tested the Pi's gpio speed and a quick search shows speeds on the order of hundreds of KHz. Not sure of the PCI 3 frequency off the top of my head but I believe each data line runs in the MHz range. 

The low frequency of the GPIO coupled with the coding overhead for each pin it would be impossible to bit bang the Pi fast enough.