This will work for numbers 0 - 2000 for large numbers you just need to cross join the original result set.
with temp as (
select distinct number
from master..spt_Values
where number between 0 and 2000
)
select * from
temp t
left join your_table y on y.col1 = t.number
where y.col1 is null
alternatively using cross join
This will work for billions obviously slower
WITH
L0 AS(SELECT 1 AS c UNION ALL SELECT 1),
L1 AS(SELECT 1 AS c FROM L0 AS A CROSS JOIN L0 AS B),
L2 AS(SELECT 1 AS c FROM L1 AS A CROSS JOIN L1 AS B),
L3 AS(SELECT 1 AS c FROM L2 AS A CROSS JOIN L2 AS B),
L4 AS(SELECT 1 AS c FROM L3 AS A CROSS JOIN L3 AS B),
L5 AS(SELECT 1 AS c FROM L4 AS A CROSS JOIN L4 AS B),
Nums AS(SELECT ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) AS n FROM L5)
select * from
l5 t
left join your_table y on y.col1 = t.n
where y.col1 is null