what is the difference between "be going to do"and"will be"?

"Be going to do" means you are intended to do something, it"s planed .
"Will be"refers to something will happen unexpected.

Hermoine is not a native speaker of English, and should not be giving advice here about the English language.

"Be going to do" is a rather awkward locution, but it suggests that someone intends to do something in the future, and i get a sense that this is uncertain. It is really difficult to answer such questions without more context. "Be going to do" is not a stock phrase for which an exclusive meaning can be given. "Will be" means that in the future, the subject will occupy a certain position or state.

Once again, offering such phrases without context makes it almost impossible to give you a reliable and coherent answer. It is much better if you can provide the sentence or the paragraph in which you read the phrase.