Goku's lifting strength is a complete unknown. Not only do we not know what the gravity of Grand Kais world is but he has also shown greater lifting while standing on the ground. Sort of a reverse Post Crisis Superman kind of deal. Might explain why goku preferred to fight on the ground than in the air. He also went from struggling with 40 tons to lifting the Z sword with one arm and holding a block of the densest metal in the universe the same way. Until we find out what those weighed we can't really judge him on this stat.
One more thing: people need to let the 40 tons thing go. It's not only an almost unusable unknown but it doesn't really affect the characters striking strength to the level it matters in comics. Going by 40 tons Goku's max lifting strength is 20,000 tons. Goku with that as his max was able to deflect with his bare hands a blast that destroyed 1/10th of the earth or around 8 moons worth of force. If his lifting strength mattered so much how the hell did he do that? He also withstood Piccolo's island clearer at point blank without budging a millimeter. Multiple mountains and the city where the tournament was held were erased and he was fine. It would take more than 20,000 tons of force to do this and he wasn't even that strong then.
Also one more thing: Gravity training and weight training are not the same. I would trade in all of my hundreds of pounds of weights for a room that went to only 2G. There is a huge difference between pressures that affect every square inch of you(Gravity Training) and those that only affect certain muscle groups which is what weight lifting is. It doesn't take much to squat 300 or more pounds or even put it on your shoulders/back and walk around. However put 40 pound ankle weights on for the first time and walking suddenly becomes far more difficult despite being a far lesser weight. I know, i've done these things in my spare time. There are also examples of this in other fictions.