Step 1 : Find prime factorization of each number.

$$\begin{aligned}36 =& 2\cdot2\cdot3\cdot3\\[8pt]54 =& 2\cdot3\cdot3\cdot3\\[8pt]90 =& 2\cdot3\cdot3\cdot5\\[8pt]126 =& 2\cdot3\cdot3\cdot7\\[8pt]9912499122 =& 2\cdot3\cdot509\cdot863\cdot3761\\[8pt]\end{aligned}$$

(view steps on how to factor 36, 54, 90, 126 and 9912499122. )

Step 2 : Put a box around factors that are common for all numbers:

$$\begin{aligned}36 =& \color{blue}{\boxed{2}}\cdot2\cdot\color{red}{\boxed{3}}\cdot3\\[8pt]54 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot3\cdot3\\[8pt]90 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot3\cdot5\\[8pt]126 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot3\cdot7\\[8pt]9912499122 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot509\cdot863\cdot3761\\[8pt]\end{aligned}$$

Step 3 : Multiply the boxed numbers together:

$$ GCD = 2\cdot3 = 6 $$

GCD Calculator