DECLARE @out TABLE(pn  VARCHAR(20), qty INT, dt  DATE);
DECLARE @in TABLE(pn   VARCHAR(20), dt   DATE, GTD  VARCHAR(20), qtyg INT);

insert @out values ('aaa', 6, '20210109'),('ccc', 2, '20210109'),('bbb', 7, '20210110'),
                   ('ccc', 8, '20210112'),('aaa', 4, '20210112'),('bbb', 3, '20210112')

insert @in values ('aaa', '20210101', 'ABC', 2),('aaa', '20210101', 'ABC2', 1),('bbb', '20210101', 'DEF', 1),
                  ('bbb', '20210101', 'DEF2', 3),('ccc', '20210101', 'JHI', 4),('aaa', '20210104', 'GKL', 2),
		  ('bbb', '20210104', 'MNO', 2),('ccc', '20210104', 'PQR', 1),('aaa', '20210107', 'STU', 3),
		  ('aaa', '20210107', 'STU2', 2),('bbb', '20210107', 'VWX', 4),('ccc', '20210107', 'YZA', 5)

;WITH nums
     AS (SELECT ROW_NUMBER() OVER(ORDER BY 1 / 0) AS n
           FROM STRING_SPLIT(REPLICATE(CAST(' ' AS VARCHAR(MAX)), 10000 - 1), ' ')),
     cte
     AS (SELECT ROW_NUMBER() OVER(PARTITION BY i.pn ORDER BY i.dt) AS rn, 
                i.pn AS pn2, i.dt AS dt2, i.GTD, x.qtyg
           FROM @in AS i
                OUTER APPLY (SELECT TOP (i.qtyg) 1 AS qtyg 
				      FROM nums
				    ORDER BY n) AS x),
     cte1
     AS (SELECT *
           FROM (SELECT *, 
                        _from = ISNULL(LAG(qty) OVER(PARTITION BY pn ORDER BY dt), 0) + 1, 
                        _to = SUM(qty) OVER(PARTITION BY pn ORDER BY dt)
                   FROM @out) AS o
          OUTER APPLY (SELECT * 
		                FROM cte AS i
		              WHERE i.pn2 = o.pn AND i.dt2 <= o.dt AND i.rn BETWEEN o._from AND o._to) AS i)
     SELECT pn, qty, dt, gtd, SUM(qtyg) AS qtyg
       FROM cte1
      GROUP BY pn, qty, dt, gtd
     ORDER BY dt, pn, gtd;

;WITH nums
     AS (SELECT ROW_NUMBER() OVER(ORDER BY 1 / 0) AS n
           FROM STRING_SPLIT(REPLICATE(CAST(' ' AS VARCHAR(MAX)), 10000 - 1), ' ')),
     cte
     AS (SELECT ROW_NUMBER() OVER(PARTITION BY i.pn ORDER BY i.dt, gtd) AS rn, 
                i.pn AS pn2, i.dt AS dt2, i.GTD, x.qtyg
           FROM @in AS i
                OUTER APPLY (SELECT TOP (i.qtyg) 1 AS qtyg 
                      FROM nums
                    ORDER BY n) AS x),
     cte1
     AS (SELECT *
           FROM ( SELECT *, _from = ISNULL(LAG(_to) OVER(PARTITION BY pn ORDER BY dt), 0) + 1 
                   FROM (SELECT *, _to = SUM(qty) OVER(PARTITION BY pn ORDER BY dt)
                          FROM @out) z) AS o
          OUTER APPLY (SELECT * 
                        FROM cte AS i
                      WHERE i.pn2 = o.pn AND i.dt2 <= o.dt AND i.rn BETWEEN o._from AND o._to) AS i)
     SELECT pn, qty, dt, gtd, SUM(qtyg) AS qtyg
       FROM cte1
      GROUP BY pn, qty, dt, gtd
     ORDER BY dt, pn, gtd;