Let p and q be any two elements of set A.
For the computation of the GCF of elements of set A, we can replace both p and q by just the GCF(p,q) and the result is unchanged.
So, for every application of the function h, we are reducing the number of elements of the set A by 1. (In this case two numbers p and q are replaced by one number GCF(p,q)).
Expanding this concept further, the minimum number of times the function h should be called is n-1.
I hope this helps!