The raw kanji is right here: この無限地獄で己の愚かさを後悔しろ= "Regret your foolishness in this infinite hell". You can also hear garlic jr say "mugen" in the video as well, so there shouldn't be any problems here. Now for the interpretation, garlic jr says to regret their decisions in the infinite hell known as the dead zone. Now I know some people will try to say, "well this could easily mean endless or infinite suffering". Well this isn't really the case here because the dead zone freezes your body and soul, so one could argue that you wouldn't really suffer at all to begin with, just slowly stay frozen until you eventually die out. And the kanji is, "無限" which literally means infinity (which in this context refers to the dead zones size). Garlic jr also said this in a way to where he is most likely talking about the dead zone itself being infinite in size. We know garlic jr also refers to the dead zone as being a hell, so "infinite hell" most likely is referring to the dead zone itself, and is not some poetic or hyperbolic way to say endless suffering, because eventually, those sucked in, like Goku and co will die in the dead zone, as even stated by garlic jr. So I think it can undoubtedly be referred to as being an infinite space in the most literal sense.