They were in Roger's era, constantly battling him. I just couldn't imagine that Garp wouldn't have heard something about it. Just his care free nature would have almost ignored it as a problem. As for Sengoku... He was a veteran for justice and I feel as though he would have known about through his rank and dedication. Akainu however is still green at being fleet admiral, so I'm gonna take a pass on him knowing and therefore everyone below him in rank knowing too.Why do you think Sengoku and Garp know about Imu?
For some weird Reason, I feel as though Kong doesn't know either.