I think most of them are, but not all of them. Although, I wouldn't be surprised if all of them have little secrets that they've been hiding from Imu, like Saturn did. I'm not sure if I count that as being "disloyal", though, personally. Saturn didn't MEAN for Emet to blow up in the World Government's face...literally. If anything, it seems like he might have been planning to study Emet in order to help the World Government. So, I'm not sure if I count that as "disloyalty".
V Nusjuro, Mars, and Warcury are probably all pretty loyal. And Qilingham also refers to Imu using a old-fashioned honorific, "ontai", meaning "boss" or "beloved leader", in chapter 1151. So, I actually think Imu is pretty beloved by those who actually know them. I don't think they're just following Imu out of fear. They might actually be pretty fond of their little leader.
It's the other two Gorosei that I'm not sure about.
Garling, like others have pointed out. He just comes across as the kind of character with ulterior motives. And if anyone's going to truly betray Imu, it's probably going to be him. Although, maybe that's just because of the glasses. The glasses make him look like he's hiding something. He really hasn't DONE anything to show he's disloyal to Imu.
But, there's also Ju Peter. This has been more subtle. But...he's been sweating...a lot. And we've been focusing on his face a few times, when he looks doubtful or afraid. Something's going on with Ju Peter. I'm not sure if it'll turn out to be that he's a total double agent or something. But, something's going on there. Oda's setting something up. I'm just not sure what. Maybe he's a traitor, or maybe he's just keeping a secret like Saturn was. But, Ju Peter's probably going to be one of the Gorosei to keep a eye on.
So, I'd say 3 out of 5 are probably pretty loyal. That's not bad. More than half anyway.