CREATE TABLE GAP(
    COUNTER INTEGER PRIMARY KEY
);

insert into gap (counter) values (1);
insert into gap (counter) values (2);
--::::: 3
--::::: 4
insert into gap (counter) values (5);
insert into gap (counter) values (6);
--::::: 7
insert into gap (counter) values (8);
insert into gap (counter) values (9);
insert into gap (counter) values (10);



--===== here the trick :

SELECT 
	list(CASE 
		WHEN N2-N1-2 > 0 THEN (N1+1) || '-'|| (N2-1)
		ELSE CAST(N1+1 AS VARCHAR(10))
	END) Missing
FROM (
SELECT a.counter N1, 
  (SELECT counter FROM gap WHERE counter > a.counter rows 1) N2 
FROM gap a
) B
WHERE N2-N1 > 1;



-- results:

Missing
=======
3-4,7