I really can't see him winning Roubaix, I doubt he will ever participate in that race again. He could possibly reach a top-10 spot, but his characteristics (sprinting on a hill) aren't really well suited to a race like Roubaix, there will always be some heavier, stronger guy who can beat him there.
If Gilbert had a team mate, yeah, that's the problem, there isn't anyone else in his team that could make the final selection on the Poggio. With Greipel, VDBjr and Gilbert they have three great riders, but the quality of the rest of the team has suffered a lot, I wouldn't be surprised of VDB and Gilbert will revisit this 'if only I had a teammate with me' thought more than they would like during this season.