WITH 
cte1 AS (
SELECT DISTINCT val - LAG(val) OVER (ORDER BY val) delta
FROM test
),
cte2 AS (
SELECT test.val, 
       cte1.delta,
       CASE WHEN LEAD(test.val) OVER (PARTITION BY cte1.delta ORDER BY test.val) - test.val > cte1.delta
            THEN 1
            ELSE 0 END new_group
FROM test
CROSS JOIN cte1
WHERE cte1.delta IS NOT NULL
),
cte3 AS (
SELECT *, 
       new_group * LEAD(new_group) OVER (PARTITION BY delta ORDER BY val) single_group
FROM cte2
),
cte4 AS (
SELECT *, SUM(single_group) OVER (PARTITION BY delta) single_groups
FROM cte3
),
cte5 AS (
SELECT CAST(MIN(delta) AS DECIMAL(10,2)) gap
FROM cte4
WHERE single_groups = 0
),
cte6 AS (
SELECT test.val,
       CASE WHEN test.val - LAG(test.val) OVER (ORDER BY test.val) > cte5.gap
            THEN 1
            ELSE 0 END new_group
FROM test
CROSS JOIN cte5
)
SELECT val, SUM(new_group) OVER (ORDER BY val) group_number
FROM cte6
WHERE val < 1000