demo, MySQL - rextester

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#MySQL 5.7.12
#please drop objects you've created at the end of the script 
#or check for their existance before creating
#'\\' is a delimiter

DROP TABLE IF EXISTS time;
CREATE TABLE time (DateTime datetime, Value int);

INSERT INTO time (DateTime, Value)
VALUES
        ('2018-08-23 00:00:00', 123),
        ('2018-08-23 00:00:05', 152),
        ('2018-08-23 00:00:10',  37),
        ('2018-08-23 00:00:15', 141),
        ('2018-08-23 00:00:20',  41),
        ('2018-08-23 00:00:25',  35),
        ('2018-08-23 00:00:30', 143),
        ('2018-08-23 00:00:35',  67),
        ('2018-08-23 00:00:40', 111),
                                        /* gap of one minute here */
        ('2018-08-23 00:01:45', 123),
        ('2018-08-23 00:01:50', 145),
        ('2018-08-23 00:01:55', 141),
        ('2018-08-23 00:02:00',  87),
                                        /* gap of 10 seconds here */
        ('2018-08-23 00:02:15', 190),
        ('2018-08-23 00:02:20', 122),
        ('2018-08-23 00:02:25', 123);

SELECT
    DateTime AS GapStart,
    NextDateTime AS GapEnd,
    TIMESTAMPDIFF(SECOND, DateTime, NextDateTime) AS SizeInSeconds
FROM
(
    SELECT DateTime, Value,
        (SELECT MIN(DateTime) FROM time t2
         WHERE t2.DateTime > t1.DateTime) AS NextDateTime
    FROM time t1
) t
WHERE
    TIMESTAMPDIFF(SECOND, DateTime, NextDateTime) > 5;   -- or whatever threshhold you want


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