Gabriel avoids Lucifer. It's easy enough to keep track of where in the world Lucifer is, his brother shines like a beacon for anyone with the capabilities to see, and Gabriel simply makes sure he isn't there. He hadn't thought it possible to be lonelier then he was when in his self-imposed exile, but with the Hosts of Heaven gone the universe seems empty and Lucifer calls to him in ways humans could never understand. He often thinks of going to his brother and at times he almost does. His brother would welcome him with open arms and a smile and that is a temptation almost too great to resist.
Yet, whenever he feels the massive weight of emptiness crush him, and he turns to fly to the beacon shining on the other side of the planet, all he can see in his mind is two eyes that had lost all hope. Sam Winchester had come to him a week before going to Detroit, tracked him down in Mexico and pleaded for his help. Gabriel had said no, he'd told Sam that he needed to say yes to Lucifer, give into destiny. Sam hadn't even bothered to argue, just left wordlessly, his eyes showing just how much Gabriel's refusal had destroyed him.
Gabriel would like to be able to say that it was Dean who fucked everything up, Dean who should've said yes and gotten the ball properly rolling. But he knows the truth and he knows he let go of something precious when he turned Sam away. He can't go to his brother because he'll see those eyes again and that would be a pain worse then the loneliness he feels to know that Sam Winchester is no longer behind them.