Step 1 : Find prime factorization of each number.

$$\begin{aligned}42 =& 2\cdot3\cdot7\\[8pt]42 =& 2\cdot3\cdot7\\[8pt]14 =& 2\cdot7\\[8pt]84 =& 2\cdot2\cdot3\cdot7\\[8pt]28 =& 2\cdot2\cdot7\\[8pt]73080 =& 2\cdot2\cdot2\cdot3\cdot3\cdot5\cdot7\cdot29\\[8pt]255780 =& 2\cdot2\cdot3\cdot3\cdot5\cdot7\cdot7\cdot29\\[8pt]\end{aligned}$$

(view steps on how to factor 42, 42, 14, 84, 28, 73080 and 255780. )

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

$$\begin{aligned}42 =& \color{blue}{\boxed{2}}\cdot3\cdot\color{red}{\boxed{7}}\\[8pt]42 =& \color{blue}{\boxed{2}}\cdot3\cdot\color{red}{\boxed{7}}\\[8pt]14 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{7}}\\[8pt]84 =& \color{blue}{\boxed{2}}\cdot2\cdot3\cdot\color{red}{\boxed{7}}\\[8pt]28 =& \color{blue}{\boxed{2}}\cdot2\cdot\color{red}{\boxed{7}}\\[8pt]73080 =& \color{blue}{\boxed{2}}\cdot2\cdot2\cdot3\cdot3\cdot5\cdot\color{red}{\boxed{7}}\cdot29\\[8pt]255780 =& \color{blue}{\boxed{2}}\cdot2\cdot3\cdot3\cdot5\cdot\color{red}{\boxed{7}}\cdot7\cdot29\\[8pt]\end{aligned}$$

Step 3 : Multiply the boxed numbers together:

$$ GCD = 2\cdot7 = 14 $$

GCD Calculator