Step 1 :
Divide $ 551 $ by $ 209 $ and get the remainder

$$ 551 : 209 = 2 \text{ ( remainder is } \color{blue}{ 133 } \text{ ) } $$

The remainder is positive ($ 133 > 0 $), so we will continue with division.

Step 2 :
Divide $ 209 $ by $ \color{blue}{ 133 } $ and get the remainder

$$ 209 : 133 = 1 \text{ ( remainder is } \color{blue}{ 76 } \text{ ) } $$

The remainder is still positive ($ 76 > 0 $), so we will continue with division.

Step 3 :
Divide $ 133 $ by $ \color{blue}{ 76 } $ and get the remainder

$$ 133 : 76 = 1 \text{ ( remainder is } \color{blue}{ 57 } \text{ ) } $$

The remainder is still positive ($ 57 > 0 $), so we will continue with division.

Step 4 :
Divide $ 76 $ by $ \color{blue}{ 57 } $ and get the remainder

$$ 76 : 57 = 1 \text{ ( remainder is } \color{blue}{ 19 } \text{ ) } $$

The remainder is still positive ($ 19 > 0 $), so we will continue with division.

Step 5 :
Divide $ 57 $ by $ \color{blue}{ 19 } $ and get the remainder

$$ 57 : \color{blue}{ \boxed { 19 }} = 3 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 19 }} $.

This solution can be visualized using a Venn diagram.

The GCD equals the product of the numbers at the intersection.

GCD Calculator