The GCD of given numbers is 1.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}5 =& 5\\[8pt]3 =& 3\\[8pt]1 =& 1~~~ \color{gray}{\text{ (1 is prime)}} \\[8pt]\end{aligned}$$(view steps on how to factor 5, 3 and 1. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}5 =& 5\\[8pt]3 =& 3\\[8pt]1 =& 1\\[8pt]\end{aligned}$$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} } $.
This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.