The major number for the gpio driver is 250.
The minor identifies the processor gpio being used, and depends on the ones you decide to create with mknod.
Some device nodes are automatically created by the boot scripts. These are platform dependant. The files containing the nodes are located under /etc/makedevs.d/ and the platform file.
Keep in mind that to use the gpio driver you need to load before the gpio module. Follow the steps described in the documentation.