Have you tried calculating the number of suitable sequences of four G's and B's as well as the number of all possible such sequences? Each suitable sequence has the form G..B with two elements in the middle.
How would you find the probability of getting the exact order of something? A couple wants to have four children. The only way they want to have those children though is if their first born is a girl and their last child born is a boy, with the gender of the two middle children not mattering. What is the probability of them having a girl first and a boy last happening?