SQL Server 2017

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CREATE TABLE tab1
(
  RoomTypeId INT,
  BedId INT,
  StartDate DATE,
  EndDate DATE
)
;

INSERT INTO tab1
(RoomTypeId, BedId, StartDate, EndDate)
VALUES 
(39, 229, '2019-08-03', '2019-08-04'),
(39, 229, '2019-08-05', '2019-08-08'),
(40, 231, '2019-08-05', '2019-08-08'),
(41, 238, '2019-08-02', '2019-08-03'),
(42, 233, '2019-08-02', '2019-08-03'),
(42, 233, '2019-08-04', '2019-08-05'),
(42, 234, '2019-08-05', '2019-08-05'),
(43, 241, '2019-08-01', '2019-08-01'),
(43, 241, '2019-08-15', '2019-08-16'),
(43, 241, '2019-08-29', '2019-08-30'),
(45, 245, '2019-08-03', '2019-08-06'),
(45, 244, '2019-08-05', '2019-08-05'),
(45, 244, '2019-08-08', '2019-08-10'),
(45, 244, '2019-08-17', '2019-08-19'),
(46, 239, '2019-08-03', '2019-08-04'),
(46, 239, '2019-08-05', '2019-08-05'),
(48, 250, '2019-08-16', '2019-08-18'),
(49, 274, '2019-08-06', '2019-08-11'),
(49, 275, '2019-08-08', '2019-08-10');

select
    main.*,
    iif(DATEDIFF(d, EndDate, nextStartDate)>0, DATEDIFF(d, EndDate, nextStartDate), 0) as GAP
from
(
    select
        RoomTypeId,
        lead(StartDate) over (partition by RoomTypeId order by StartDate asc) as nextStartDate,
        StartDate,
        EndDate
    from 
        tab1
) main;

RoomTypeId	nextStartDate	StartDate	EndDate	GAP
39	05/08/2019 00:00:00	03/08/2019 00:00:00	04/08/2019 00:00:00	1
39	null	05/08/2019 00:00:00	08/08/2019 00:00:00	0
40	null	05/08/2019 00:00:00	08/08/2019 00:00:00	0
41	null	02/08/2019 00:00:00	03/08/2019 00:00:00	0
42	04/08/2019 00:00:00	02/08/2019 00:00:00	03/08/2019 00:00:00	1
42	05/08/2019 00:00:00	04/08/2019 00:00:00	05/08/2019 00:00:00	0
42	null	05/08/2019 00:00:00	05/08/2019 00:00:00	0
43	15/08/2019 00:00:00	01/08/2019 00:00:00	01/08/2019 00:00:00	14
43	29/08/2019 00:00:00	15/08/2019 00:00:00	16/08/2019 00:00:00	13
43	null	29/08/2019 00:00:00	30/08/2019 00:00:00	0
45	05/08/2019 00:00:00	03/08/2019 00:00:00	06/08/2019 00:00:00	0
45	08/08/2019 00:00:00	05/08/2019 00:00:00	05/08/2019 00:00:00	3
45	17/08/2019 00:00:00	08/08/2019 00:00:00	10/08/2019 00:00:00	7
45	null	17/08/2019 00:00:00	19/08/2019 00:00:00	0
46	05/08/2019 00:00:00	03/08/2019 00:00:00	04/08/2019 00:00:00	1
46	null	05/08/2019 00:00:00	05/08/2019 00:00:00	0
48	null	16/08/2019 00:00:00	18/08/2019 00:00:00	0
49	08/08/2019 00:00:00	06/08/2019 00:00:00	11/08/2019 00:00:00	0
49	null	08/08/2019 00:00:00	10/08/2019 00:00:00	0