Gohan was the strongest in his mystic form back at the end of DBZ.
But in Dragon Ball Super Series Gohan have become so weak that he lost the ability to turn Super Saiyan 2 in such a short period of time.
The theory acts actually goes like this, Mystic Gohan got his powers without training (Old Ki awakened his powers). After the Kid Buu incident and Berus arrival approximately 5.5 years had passed, Mystic Gohan didn’t train for 5 whole years, he didn’t even use his KI for that much time. At first he got the power as a given and after that he didn’t train so his power drained off in the first place.The amount of time that’s passed in Dragon Ball Super is apparently open to interpretation.
In BoG, Gohan was probably still the strongest or at least on the same level as Goku before he goes SSJG, hence the reason why they made it a point to have him step in as if he was the only one who could stop Beerus. Obviously, that didn’t work out too well for him. But he powered up into his “mystic” form (I still don’t understand why people call it that…) and that’s the last time we see it, so far.