The GCD of given numbers is 6.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}18 =& 2\cdot3\cdot3\\[8pt]2436 =& 2\cdot2\cdot3\cdot7\cdot29\\[8pt]54 =& 2\cdot3\cdot3\cdot3\\[8pt]90 =& 2\cdot3\cdot3\cdot5\\[8pt]126 =& 2\cdot3\cdot3\cdot7\\[8pt]\end{aligned}$$(view steps on how to factor 18, 2436, 54, 90 and 126. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}18 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot3\\[8pt]2436 =& \color{blue}{\boxed{2}}\cdot2\cdot\color{red}{\boxed{3}}\cdot7\cdot29\\[8pt]54 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot3\cdot3\\[8pt]90 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot3\cdot5\\[8pt]126 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot3\cdot7\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 2\cdot3 = 6 $$