Before installing debian, You need to prepare tftp server and connect to guruplug via serial console. Also You’ll need UTP cable for network connection and SD card for installing the base system.
The first thing you have to do, is to read this: http://loginroot.com/disclaimer/ Before installing uboot to guruplug server plus you have to set few things. You have to be able to connect to Guruplug via serial connection. The second thing, is to have tftp server up and running. Also you need to have utp cable (to transfer the image to guruplug) and the boot image itself.
If you cannot connect to guruplug for some reason via lan interface, for example sshd stopped running or just firewalled yourself out of it, you can connect to it using serial connection.