user_number	|    seq_number
1	        |       101
1	        |       102
1		|	109
1		|	110
1		|	111
2		|	201
2		|	202
2		|	203
2		|	207
2		|	208
.		|	.
.		|	.
.		|	.
.		|	.

select gap_start - gap_end gap
from(
select a.seq_number+1 as gap_start, b.seq_number-1 as gap_end 
from   my_table a, my_table b
where a.seq_number = b.seq_number  
and a.seq_number - (select c.seq_number 
                    from my_table c
                    where c.seq_number = a.seq_number-1) is null
and a.user_number = '1'
order by a.seq_number
);

SQL> WITH DATA AS(
  2  SELECT 1    user_number, 101 seq_number FROM dual UNION ALL
  3  SELECT 1,      102 FROM dual UNION ALL
  4  SELECT 1,                  109 FROM dual UNION ALL
  5  SELECT 1,                  110 FROM dual UNION ALL
  6  SELECT 1,                  111 FROM dual UNION ALL
  7  SELECT 2,                  201 FROM dual UNION ALL
  8  SELECT 2,                  202 from dual union all
  9  SELECT 2,                  203 FROM dual UNION ALL
 10  SELECT 2,                  207 FROM dual UNION ALL
 11  SELECT 2,                  208 FROM dual)
 12  SELECT user_number,
 13    seq_number,
 14    seq_number - rn diff
 15  FROM
 16    (SELECT user_number,
 17      seq_number,
 18      lag(seq_number) OVER(partition by user_number ORDER BY user_number, seq_number) rn
 19    FROM DATA
 20    )
 21  WHERE seq_number - rn > 1
 22  /

USER_NUMBER SEQ_NUMBER       DIFF
----------- ---------- ----------
          1        109          7
          2        207          4

SQL>

 CREATE OR replace PROCEDURE P_validate_seq (p_gaps_alowed IN NUMBER DEFAULT 1)
IS
  --get the distinct user number
  CURSOR c_user_number IS
    SELECT DISTINCT user_number
    FROM   advices
    WHERE  action_type = 0;
  r_user_number c_user_number%ROWTYPE;
  --calculate the gaps between the sequences based on user number
  CURSOR c_advice_seq (
    p_user_number advices.user_number%TYPE) IS
    SELECT ( advice_sequence_number - rn ) gap
    FROM   (SELECT user_number,
                   action_type,
                   advice_sequence_number,
                   Lag (advice_sequence_number)
                     over (
                       PARTITION BY user_number
                       ORDER BY user_number, advice_sequence_number) rn
            FROM   advices)
    WHERE  advice_sequence_number - rn > 1
           AND user_number = p_user_number
           AND action_type = 0;
  r_advice_seq  c_advice_seq%ROWTYPE;
BEGIN
    FOR r_user_number IN c_user_number LOOP
        FOR r_advice_seq IN c_advice_seq (r_user_number.user_number) LOOP
            IF r_advice_seq.gap > p_gaps_alowed THEN
              dbms_output.Put_line (
              'sequence number should be sequential per user number: '
              || r_user_number.user_number);
            END IF;
        END LOOP;
    END LOOP;
EXCEPTION
  WHEN OTHERS THEN
             g_routine_name := 'p_validate_seq';
END p_validate_seq;

SQL>  CREATE OR replace PROCEDURE P_validate_seq (p_gaps_alowed IN NUMBER DEFAULT 1)
  2  IS
  3    --get the distinct user number
  4    CURSOR c_user_number IS
  5      SELECT DISTINCT user_number
  6      FROM   advices
  7      WHERE  action_type = 0;
  8    r_user_number c_user_number%ROWTYPE;
  9    --calculate the gaps between the sequences based on user number
 10    CURSOR c_advice_seq (
 11      p_user_number advices.user_number%TYPE) IS
 12      SELECT ( advice_sequence_number - rn ) gap
 13      FROM   (SELECT user_number,
 14                     action_type,
 15                     advice_sequence_number,
 16                     Lag (advice_sequence_number)
 17                       over (
 18                         PARTITION BY user_number
 19                         ORDER BY user_number, advice_sequence_number) rn
 20              FROM   advices)
 21      WHERE  advice_sequence_number - rn > 1
 22             AND user_number = p_user_number
 23             AND action_type = 0;
 24    r_advice_seq  c_advice_seq%ROWTYPE;
 25  BEGIN
 26      FOR r_user_number IN c_user_number LOOP
 27          FOR r_advice_seq IN c_advice_seq (r_user_number.user_number) LOOP
 28              IF r_advice_seq.gap > p_gaps_alowed THEN
 29                dbms_output.Put_line (
 30                'sequence number should be sequential per user number: '
 31                || r_user_number.user_number);
 32              END IF;
 33          END LOOP;
 34      END LOOP;
 35  END p_validate_seq;  
 36  /

Warning: Procedure created with compilation errors.

SQL> show errors;
Errors for PROCEDURE P_VALIDATE_SEQ:

LINE/COL ERROR
-------- -----------------------------------------------------------------
17/27    PLS-00103: Encountered the symbol "(" when expecting one of the
         following:
         , from

select * from v$version;

SQL>  CREATE OR replace PROCEDURE P_validate_seq (p_gaps_alowed IN NUMBER DEFAULT 1)
  2  IS
  3    --get the distinct user number
  4    CURSOR c_user_number IS
  5      SELECT DISTINCT user_number
  6      FROM   advices
  7      WHERE  action_type = 0;
  8    r_user_number c_user_number%ROWTYPE;
  9    --calculate the gaps between the sequences based on user number
 10    CURSOR c_advice_seq (
 11      p_user_number advices.user_number%TYPE) IS
 12      SELECT ( advice_sequence_number - rn ) gap
 13      FROM   (SELECT user_number,
 14                     action_type,
 15                     advice_sequence_number,
 16                     Lag (advice_sequence_number)
 17                       over (
 18                         PARTITION BY user_number
 19                         ORDER BY user_number, advice_sequence_number) rn
 20              FROM   advices)
 21      WHERE  advice_sequence_number - rn > 1
 22             AND user_number = p_user_number
 23             AND action_type = 0;
 24    r_advice_seq  c_advice_seq%ROWTYPE;
 25  BEGIN
 26      FOR r_user_number IN c_user_number LOOP
 27          FOR r_advice_seq IN c_advice_seq (r_user_number.user_number) LOOP
 28              IF r_advice_seq.gap > p_gaps_alowed THEN
 29                dbms_output.Put_line (
 30                'sequence number should be sequential per user number: '
 31                || r_user_number.user_number);
 32              END IF;
 33          END LOOP;
 34      END LOOP;
 35  END p_validate_seq;  
 36  /