WARNING: THIS ANSWER CONTAINS SPOILERS UP TO EPISODE 23
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 and that's the last time we see it, so far.
I don't see how he could have fallen so far behind Goku and Vegeta prior to their being trained by Whis, but basically Gohan hasn't trained, like at all. Not like the time between when he fought Cell and Majin Buu where he just slacks on his training—this time he really did nothing at all. The most "training" you see him do is when he's play fighting with Mr. Satan as Great Saiyaman over baby Pan.
They do make it a point to show that he hasn't necessarily lost his power, as he's able to easily beat Ginyu (in Tagoma's body) as a Super Saiyan, just the ability to access all of it at will without seriously injuring or killing himself has been lost. Against Frieza, he may have been able to fight with him and at least hold him off till Goku and Vegeta got there, but we all saw what bringing out more power than his body can handle did to him. Think Kaioken and what happened to Goku when he fought Vegeta, except on a much larger scale.
They also made it a point to show that maybe he wasn't necessarily "too weak" to face Frieza, just too soft. He lets his guard down while showing mercy to Ginyu in the same way Goku did with Frieza and pays for it with a ki blast through the shoulder. Given that he was clearly angry with himself for letting his training go, which made him unable to protect his family, maybe he's learned his lesson about just assuming his dad or Vegeta will always be there to protect everyone and will find some motivation to train a little bit. We'll see. My point is that they seem to be putting raw power into a grey area and focusing more on ability, experience and fighting spirit in this series. That's just my opinion based on the fact that Roshi was there fighting when Yamcha is easily at least still stronger than he is. Anyway, I hope that helps anyone wondering what's happening with Gohan. He's not weak, power-wise, he's just out of shape and rocking the dad bod.