sql>select * from t order by c;
 
        C
---------
        1
        2
        4
        5
        8
 
5 rows selected.
 
sql>select (c + 1) gap_start, (next_num - 1) gap_end
  2    from (select c, lead(c) over (order by c) next_num
  3            from t)
  4   where (c + 1) <> next_num;
 
GAP_START   GAP_END
--------- ---------
        3         3
        6         7
 
2 rows selected.