It should be 'whom', not 'whose'. Technically it should also be 'whom' in the original question in your text, but 'whom' is falling out of use.

I would say "Brenda is who Gary is used to asking for help." If it's from a work book I would use as much of the question in the answer as possible, but it is good to think of more ways to say something! In English we only say "question" we do not say doubt in this case. When you say doubt it is always negative. "I have doubts about this project" = I don't have faith in this project "I have questions about this project" = I want to know more about this project.

It isn't incorrect, but when answering a question it is best to repeat the question in your answer. (to make certain you answer that was is asked) So in this case, I would definitely go for the 'simple' way.^^ (probably 100% of the time, especially since it's a textbook question and not a verbal conversation)