Gamma Printer Error

Hello, I need help. While I want to setup a printer, nothing happen and this is spammed in the server console :

[ERROR] addons/gamma_printer_19a/lua/entities/gamma_printer/init.lua:120: attempt to call global 'GP_Think' (a nil value)

1. unknown - addons/gamma_printer_19a/lua/entities/gamma_printer/init.lua:120

So i go to the init.lua file and, at the line 120, here is what there is :

function ENT:Think()

   GP_Think(self)

   self:NextThink(CurTime()+self.Interval)

   return true

end

Did you know where did this come from and how to solve this ? (Sorry if my english is bad, i'm french)