No, that’s a gigantic difference, but it’s also the worst-case scenario. In a real-world scenario the effect wouldn’t be nearly as dramatic plus you’d have level designers and artists do proper lighting meaning that the “before” pic would hardly be that bad. I am not saying that GI doesn’t make a huge difference in “laboratory” scenarios but if you take one finished scene, or a finished game even, and toggle GI on and off like in the pic above, the difference would hardly be that striking.