I will gradually supply answers.
Write an algorithm for the division algorithm and determine number of steps (in terms of inputs). Then find a divide and conquer algorithm to compute gcd(a,b). Compute it's complexity using 5.3 where you could set f(n) to be the maximum number of steps needed to compute gcd(a,b) for any a,b at most n. You should be able to get f(n)=f(n/2) + g(n) (with g(n) not too big) by going two steps at a time.