Gr-extras tun/tap

Hi
Can any one please explain how this gr-extras tun/tap device can be
used. I need an example of how to use this device in an application. I
already know the Linux documentation
https://www.kernel.org/doc/Documentation/networking/tuntap.txt

for the tun/tap device but can not connect it to my application to talk
to
this device.

Any example will be of help.

Regards

Guru