Cobol implementation of https://www.youtube.com/watch?v=UVUjnzpQKUo&lc=Ugw5QFQbjIvZmQ89o6B4AaABAg

gcd_maxmin.cob

       *> Compile using GNU Cobol, e.g.: `cobc -x gcd_maxmin.cob`
       IDENTIFICATION DIVISION.
       PROGRAM-ID. gcd-maxmin.
       ENVIRONMENT DIVISION.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01 NUMS.
           05 NUMS-ELEMS PIC 9(9)
              OCCURS 0 TO 9999 TIMES DEPENDING ON NUMS-LEN.
       77 NUMS-IDX PIC 9(9) COMP.
       77 NUMS-LEN PIC 9(9).
       01 MAX-NUM PIC 9(9).
       01 MIN-NUM PIC 9(9).
       01 RESULT PIC 9(9).

       PROCEDURE DIVISION.
       MAIN-LINE.
           DISPLAY "Enter size of `nums`:".
           ACCEPT NUMS-LEN.
           DISPLAY "Enter elements of `nums`:".
           PERFORM
               VARYING NUMS-IDX FROM 1 BY 1
               UNTIL NUMS-IDX = NUMS-LEN + 1
           ACCEPT NUMS-ELEMS(NUMS-IDX) END-PERFORM.

           CALL "ARRAY-MINMAX" USING
              BY REFERENCE NUMS,
              NUMS-LEN,
              MAX-NUM,
              MIN-NUM.

           CALL "GCD" USING
              BY CONTENT MIN-NUM,
              BY CONTENT MAX-NUM,
              BY REFERENCE RESULT.

           DISPLAY RESULT.
       GOBACK.

       *> Given an `ARRAY`  of `ARRAY-LEN` elements,
       *> will return the maximum and minimum value in the array.
       *> In some COBOLs there is a shorthand to run an intrinsic function directly on a whole array
       *> (e.g.: `COMPUTE MAX-RES = FUNCTION MAX(ARRAY-ELEMS(ALL))` ) but this is not supported in GNU Cobol.
       IDENTIFICATION DIVISION.
       PROGRAM-ID. ARRAY-MINMAX.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01 IDX PIC 9(9) COMP.

       LINKAGE SECTION.
       01 ARRAY.
           05 ARRAY-ELEMS PIC 9(9)
              OCCURS 0 TO 9999 TIMES DEPENDING ON ARRAY-LEN.
       77 ARRAY-LEN PIC 9(9).
       01 MAX-RES PIC 9(9).
       01 MIN-RES PIC 9(9).
       PROCEDURE DIVISION USING ARRAY ARRAY-LEN MAX-RES MIN-RES.
               MOVE ARRAY-ELEMS(1) TO MAX-RES.
               MOVE ARRAY-ELEMS(1) TO MIN-RES.
               DISPLAY MIN-RES.
               DISPLAY MAX-RES.
               PERFORM VARYING IDX FROM 2 BY 1
                       UNTIL IDX >= ARRAY-LEN + 1
                   COMPUTE MAX-RES =
                       FUNCTION MAX(MAX-RES, ARRAY-ELEMS(IDX))
                   COMPUTE MIN-RES =
                       FUNCTION MIN(MIN-RES, ARRAY-ELEMS(IDX))
               END-PERFORM.
       GOBACK.
       END PROGRAM ARRAY-MINMAX.

       *> Implementation of the Euclidean Algorithm.
       IDENTIFICATION DIVISION.
       PROGRAM-ID. GCD.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01 TEMP PIC 9(9).
       LINKAGE SECTION.
       01 A PIC 9(9).
       01 B PIC 9(9).
       01 RESULT PIC 9(9).

       PROCEDURE DIVISION USING A, B, RESULT.
           PERFORM UNTIL B = 0
               MOVE A TO TEMP
               MOVE B TO A
               DIVIDE TEMP BY B GIVING TEMP REMAINDER B
           END-PERFORM.
               MOVE A TO RESULT.
       GOBACK.
       END PROGRAM GCD.

This code was written by me. After a Hello World I wrote a couple of months back, this is my second program written in Cobol.
While I am fairly certain the code is somewhat idiomatic (if one can say such things about code written in Cobol at all), I am by no means a Cobol expert so it might be possible to improve this code.