SQL Server 2019

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CREATE TABLE dbo.GLProject
(project int, amount int, period date);

INSERT dbo.GLProject VALUES
(12345,	10, '20200101'),
(12345,	10, '20200201'),
(12345,	10, '20200301'),
(12345,	10, '20200401'),
(12345,	10, '20200501'),
(12345,	10, '20200601'),
(12345, 24, '20200901')/*,
(23456, 31, '20200101'),
(23456, 55, '20201101')*/;

DECLARE @d date = DATEFROMPARTS(YEAR(getdate()), MONTH(getdate()), 1);

;WITH total_range AS 
(
  SELECT project, period = MIN(period)
    FROM dbo.GLProject
    GROUP BY project
  UNION ALL 
  SELECT project, DATEADD(MONTH, 1, period)
  FROM total_range 
  WHERE period < @d
)
SELECT 
  tr.project, 
  amount = SUM(COALESCE(p.amount,0)) OVER 
           (PARTITION BY tr.project ORDER BY tr.period), 
  period = CONVERT(char(6), tr.period, 112)
FROM total_range AS tr
LEFT OUTER JOIN dbo.GLProject AS p 
ON tr.project = p.project
AND tr.period = p.period;

;WITH total_range AS 
(
  SELECT project, period = MIN(period)
    FROM dbo.GLProject
    GROUP BY project
  UNION ALL 
  SELECT project, DATEADD(MONTH, 1, period)
  FROM total_range 
  WHERE period < @d
)
SELECT 
  tr.project, 
  amount = SUM(COALESCE(p.amount,0)) OVER 
           (PARTITION BY tr.project ORDER BY tr.period
            ROWS UNBOUNDED PRECEDING), 
  period = CONVERT(char(6), tr.period, 112)
FROM total_range AS tr
LEFT OUTER JOIN dbo.GLProject AS p 
ON tr.project = p.project
AND tr.period = p.period;

project	amount	period
12345	10	202001
12345	20	202002
12345	30	202003
12345	40	202004
12345	50	202005
12345	60	202006
12345	60	202007
12345	60	202008
12345	84	202009
12345	84	202010
12345	84	202011
12345	84	202012
12345	84	202101
12345	84	202102
12345	84	202103
12345	84	202104
12345	84	202105
12345	84	202106
12345	84	202107
12345	84	202108
12345	84	202109
12345	84	202110
12345	84	202111
12345	84	202112
12345	84	202201
12345	84	202202
12345	84	202203
12345	84	202204
12345	84	202205
12345	84	202206
12345	84	202207
12345	84	202208
12345	84	202209
12345	84	202210
12345	84	202211
12345	84	202212
12345	84	202301
12345	84	202302
12345	84	202303
12345	84	202304
12345	84	202305
12345	84	202306
12345	84	202307
12345	84	202308
12345	84	202309
12345	84	202310
12345	84	202311
12345	84	202312
12345	84	202401

project	amount	period
12345	10	202001
12345	20	202002
12345	30	202003
12345	40	202004
12345	50	202005
12345	60	202006
12345	60	202007
12345	60	202008
12345	84	202009
12345	84	202010
12345	84	202011
12345	84	202012
12345	84	202101
12345	84	202102
12345	84	202103
12345	84	202104
12345	84	202105
12345	84	202106
12345	84	202107
12345	84	202108
12345	84	202109
12345	84	202110
12345	84	202111
12345	84	202112
12345	84	202201
12345	84	202202
12345	84	202203
12345	84	202204
12345	84	202205
12345	84	202206
12345	84	202207
12345	84	202208
12345	84	202209
12345	84	202210
12345	84	202211
12345	84	202212
12345	84	202301
12345	84	202302
12345	84	202303
12345	84	202304
12345	84	202305
12345	84	202306
12345	84	202307
12345	84	202308
12345	84	202309
12345	84	202310
12345	84	202311
12345	84	202312
12345	84	202401