feat: add gcd at the end of the file

src/calculator.py

@@ -12,3 +12,9 @@ def modulo(left: int, right: int) -> int:
     if right == 0:
         raise ValueError('modulo by zero')
     return left % right
+
+
+def gcd(left: int, right: int) -> int:
+    while right:
+        left, right = right, left % right
+    return abs(left)