The GCD of given numbers is 1.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}40 =& 2\cdot2\cdot2\cdot5\\[8pt]6 =& 2\cdot3\\[8pt]43 =& 43\\[8pt]5 =& 5\\[8pt]42 =& 2\cdot3\cdot7\\[8pt]2 =& 2\\[8pt]6 =& 2\cdot3\\[8pt]6 =& 2\cdot3\\[8pt]12 =& 2\cdot2\cdot3\\[8pt]1 =& 1~~~ \color{gray}{\text{ (1 is prime)}} \\[8pt]1 =& 1~~~ \color{gray}{\text{ (1 is prime)}} \\[8pt]1 =& 1~~~ \color{gray}{\text{ (1 is prime)}} \\[8pt]\end{aligned}$$(view steps on how to factor 40, 6, 43, 5, 42, 2, 6, 6, 12, 1, 1 and 1. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}40 =& 2\cdot2\cdot2\cdot5\\[8pt]6 =& 2\cdot3\\[8pt]43 =& 43\\[8pt]5 =& 5\\[8pt]42 =& 2\cdot3\cdot7\\[8pt]2 =& 2\\[8pt]6 =& 2\cdot3\\[8pt]6 =& 2\cdot3\\[8pt]12 =& 2\cdot2\cdot3\\[8pt]1 =& 1\\[8pt]1 =& 1\\[8pt]1 =& 1\\[8pt]\end{aligned}$$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} } $.