To factor the expression \(9z + 18\) using the Greatest Common Factor (GCF), we first need to identify the GCF of the coefficients in the expression.
The coefficients are 9 and 18. The GCF of 9 and 18 is 9.
Now, we factor out the GCF from the expression:
\[ 9z + 18 = 9(z + 2) \]
So, the factored form of \(9z + 18\) is:
\[ \boxed{9(z + 2)} \]