Question

why did Gopal disguise himself when he went to buy a hilsa fish

Answer 1

he did that because he had taken up a challenge of making kingdom not talk about the hilsa fish but for something else . so he disguised himself in such a way that everyone would talk bout him not the hilsa fish

Answer 2

Actually the KING challenged GOPAL, about bringing a HILSA FISH, without being interrogated about it by anyone, as the KING himself, was unable to do so.

That's why GOPAL DISGUISED himself, so that everybody thinks that he is a MAD person, and no one asks him anything about the HILSA FISH in his hand...