bilal skaf has reportedly been attacked by inmates in goulburn prison yard .
the gang rape leader was assaulted by three other inmates on friday .
he was treated in hospital for head injuries and returned to prison .
bilal skaf was sentenced a maximum 55 years in prison in 2002 for gang raping young girls in sydney 's southwest when he was just 18 .
he is not eligible for parole until 2033 and shares a cell with his brother mohammed .