The GCD of given numbers is 1.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}503 =& 503\\[8pt]1101 =& 3\cdot367\\[8pt]2002 =& 2\cdot7\cdot11\cdot13\\[8pt]\end{aligned}$$(view steps on how to factor 503, 1101 and 2002. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}503 =& 503\\[8pt]1101 =& 3\cdot367\\[8pt]2002 =& 2\cdot7\cdot11\cdot13\\[8pt]\end{aligned}$$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.