I have heard that some people got GC from third party sponsor, and I don't know what does that mean. Does anyone know here, what does it mean, and if it evens possible?

Yes that is true you can get GC from third party sponsor. It mean if your spouse can’t sponsor you or don’t make enough money to sponsor you then someone else who is willing to sponsor you can co-sponsor you and give their income details and w2 while filing for GC with other necessary documents along with sponsorship form. One of my friend got GC just like that.