Why is Gohan shown to be so weak in Dragon Ball Super? Gohan was the strongest in his mystic form back at the end of DBZ. How could Gohan have become so weak that he lost the ability to turn Super Saiyan 2 in such a short period of time? While on the other hand, Frieza was able to surpass Goku's Super Saiyan Blue transformation in only 4 months of training. That doesn't make sense.

Tagoma was even able to pierce him with a ki blast. Now he is losing left and right in every episode. Most recently Frieza in his base form was shooting ki blasts through Super Saiyan Gohan. So Frieza's base form is stronger than Super Saiyan now? How did that happen?
How could be become so weak that cant even go super saiyan 2 anymore in a short period of time


According to the timeline of dragon ball, about 5 years have passed between the end the buu saga and the Resurection of F saga. Gohan hasn't trained at all since then. We can choose the analogy of professional athletes here: they need to train almost everyday to maintain their form, let alone get "stronger". After 5 years, it is expected than Gohan has become significantly weaker. But again, if he were to train for a few months, his power would probably go up exponentially. Also note that he did not train much in the Buu saga, much of his power increase came from the help of Supreme Kai.


  ...while frieza surpasses goku SSB in 4 months of training


Frieza states that he never had to train one day in his life because he was already so much stronger than everybody else. If we consider Goku to be a combat genius, imagine how much training and near-deaths experience he had to go through to be able to fight on par with Frieza. In the end, only Krilin's death helped become super saiyan and beat Frieza. Since he never trained before, it would make sense that his power would go up exponentially after little training. 


  Tagoma was even able to pierce him with a ki blast.


In the movie "Resurrection of F" Goku in Super Saiyan blue was almost killed by a Sorbet's gun. This tells you that no matter how strong of a warrior you are, you can be defeated by someone so much weaker than you if you are caught by surprise.  


  So frieza base is stronger than super saiyan now


Yes, when Gohan clearly states than he is no match of Frieza, he is still in his base form. Gohan might be able to sense Frieza's ki beyond his current form, but this is just speculation.