When Goku and Gohan are in the hyperbolic time chamber, you see them talking. At one point, the camera zooms out over their heads, and you'll see Gohan's mouth moving when Goku is talking.
thats a little thing called "dubbing". in Japan, gohan probably says something, but it was edited out for some reason, maybe to make the episode shorter. otherwise, a mistake was made and gohan was supposed to say that, but the translators thought it was for gokuh to say. do you understand now? if not, then think of this. in Japan, the words are MUCH longer than in america. maybe he was done talking in English, but the Japanese words were still going. now do you understand?
I have to say something about the dubbing. Funimation does a good job on the dubbing (only on the animation. The choice of translation is a different story). They change the animation of the mouths to match the words spoken. If the voices spoken were true then it would be a genuine slip up.