The GRB starts, from my understanding, when the core collapses - and that only takes seconds. And then travels at light speed. The shockwave making its way through the rest of the star can take hours, and you might see the explosion take months. But the GRB would hit us before we would see the shockwave, or explosion.

Without the ability to accurately determine when a supernova will occur, we will effectively have no real warning we're about to be hit.

Protecting ourselves from it is also well beyond our current technology. We'd either need to build enough physical shielding to block it from hitting earth, or we'd need to leave our solar system.