The gap is where the fun happens. The gap is where business analysis is more art than science.
If we have a well-defined current state and a well-defined future state there are many ways of getting from one to the other.
Picking the optimum path means the business analyst has to understand the stakeholders involved and their strengths and weaknesses.
Picking the optimum path means the business analyst has to understand the organization and its strengths and weakness.
Picking the optimum path means the business analyst has to understand the available resources and assets involved and their strengths and weakness.
Picking the optimum path means the business analyst has to understand how all of the above contribute to risks and how to mitigate those risks.