I think it was Gyp.
Definitely Gyp. Gillian (I think, may have been someone else) told Gyp that Nucky and Rothstein were going to be eating there and the time that they would be there.
One thing that I don't think was mentioned, but Nucky/Rothstein were talking about hiring Chinese people to do something (I think run the distillery) and some Asian guy walked right behind Nucky just before the explosion as he was looking at Billie. That wouldn't usually mean much, but I'm pretty sure that it was both the first time Chinese people were mentioned and the first time one was shown. Wondering if the people that Nucky plans to hire are already working for Gyp.