@stenfano actually gohan transformed to super sayain 1 and goku was haveing trouble lifting the sword (more so then gohan)
I'm pretty sure that he was in SSj2.
Yeah, Goku was haveing trouble lifting the sword, but the same was for Gohan initially.
The strength difference between Gohan and Goku before the Zeta-Training was not so big, but Goku is likely the strongest. Goku obviously, uses full effort to hold the sword since he struggles to lift it nor to swing it properly without falling of balance.
You may ask, why has that got to do with "full effort". Easy actually. Gohan´s full effort was enough to swing the sword, not as much as Goku but at least once. While Goku on the above picture seems to struggle he at least had an strength advantage over Gohan during that time, since he was still able to swing the sword more then Gohan.
Again. Gohan trains with his Super-Saiyan to gain the strength he needs to train with it properly.
Gohan is now strong enough in base, to lift it like his Super-Saiyan state.
And here we are, Gohan now has the SKILL and the STRENGTH to lift the sword properly and even beyond he mastered it as it seems.