Gabriel was nowhere to be found after the last time the Winchesters saw him. That was when he got 'killed' by Lucifer. He didn't really get killed, just sent of somewhere that he couldn't get out of. He didn't know how much time had past since he ended up there, when finally he saw something that wasn't ever there before. He went toward it, and suddenly was standing over burnt wings on the floor of the restaurant that Lucifer killed him in.
Sam Winchester sat at his laptop, trying to find any clues that he came back. Castiel was able to come back, why not Gabriel? As he sat there, he quickly closed everything that showed he was looking for him as the Impala pulled up outside. He didn't care if accused him of looking at porn or something, not wanting to tell him he has been searching for years for Gabe. When Dean walked in, Sam was instead looking for a case, since they just finished the one they were in town for. He found one at the restaurant that the Gods met up at, where Gabriel was killed. A witness said that there was a bright light and as they went in there was no one there.