The GCD of given numbers is 1.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}88 =& 2\cdot2\cdot2\cdot11\\[8pt]9 =& 3\cdot3\\[8pt]88 =& 2\cdot2\cdot2\cdot11\\[8pt]2 =& 2\\[8pt]7 =& 7\\[8pt]\end{aligned}$$(view steps on how to factor 88, 9, 88, 2 and 7. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}88 =& 2\cdot2\cdot2\cdot11\\[8pt]9 =& 3\cdot3\\[8pt]88 =& 2\cdot2\cdot2\cdot11\\[8pt]2 =& 2\\[8pt]7 =& 7\\[8pt]\end{aligned}$$Note that in this example numbers do not have any common factors.
Step 3 : Multiply the boxed numbers together:
Since there is no boxed numbers we conclude that $~\color{blue}{ \text{GCD = 1} } $.