SQL Server 2019

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

declare @date datetime2(7);
declare @period int;
set @date = '20210224';
set @period = 30;

declare @raw_data table(GatewayName varchar(23), StartDateTime datetime2(7), DisconnectDateTime datetime2(7), ConnectionDuration int , Trunk varchar(10));
insert @raw_data (GatewayName, StartDateTime, DisconnectDateTime, ConnectionDuration, Trunk) values
('GW1', '2021-02-24 20:01:00.0', '2021-02-24 20:05:30.0', DATEDIFF(millisecond, '2021-02-24 20:01:00.0', '2021-02-24 20:05:30.0'), 'T1' ),
('GW1', '2021-02-24 20:05:20.6', '2021-02-24 20:07:50.1', DATEDIFF(millisecond, '2021-02-24 20:05:20.6', '2021-02-24 20:07:50.1'), 'T1' ),
('GW1', '2021-02-24 20:04:50.0', '2021-02-24 20:08:24.0', DATEDIFF(millisecond, '2021-02-24 20:04:50.0', '2021-02-24 20:08:24.0'), 'T1' ),
('GW1', '2021-02-24 20:15:50.0', '2021-02-24 20:17:00.0', DATEDIFF(millisecond, '2021-02-24 20:15:50.0', '2021-02-24 20:17:00.0'), 'T1' ),
('GW1', '2021-02-24 20:20:50.0', '2021-02-24 20:21:00.0', DATEDIFF(millisecond, '2021-02-24 20:20:50.0', '2021-02-24 20:21:00.0'), 'T1' ),
('GW1', '2021-02-24 20:20:59.0', '2021-02-24 20:24:00.0', DATEDIFF(millisecond, '2021-02-24 20:20:59.0', '2021-02-24 20:24:00.0'), 'T1' ),
('GW1', '2021-02-24 20:25:00.0', '2021-02-24 20:28:30.0', DATEDIFF(millisecond, '2021-02-24 20:25:00.0', '2021-02-24 20:28:30.0'), 'T1' ),
('GW2', '2021-02-24 20:41:49.0 ', '2021-02-24 20:43:24.0', DATEDIFF(millisecond, '2021-02-24 20:41:49.0 ', '2021-02-24 20:43:24.0'), 'T2' ),
('GW3', '2021-02-24 22:46:54.2', '2021-02-24 22:48:25.2', DATEDIFF(millisecond, '2021-02-24 22:46:54.2', '2021-02-24 22:48:25.2'), 'T1' ),
('GW99', '2021-02-24 22:47:25.1', '2021-02-24 22:47:54.4', DATEDIFF(millisecond, '2021-02-24 22:47:25.1', '2021-02-24 22:47:54.4'), 'T2' )
;
select * from @raw_data;

with numbers(val) as 
    (select 1 union all select val + 1 from numbers where val < 48)
select @date, nbr.val, 
   dateadd(minute, (nbr.val - 1) * 30, @date) as period_start, 
   dateadd(minute, (nbr.val    ) * 30, @date) as period_end 
from numbers as nbr 
order by nbr.val;

with numbers(val) as 
    (select 1 union all select val + 1 from numbers where val < 48),
periods as (
    select @date as [date], nbr.val, 
   dateadd(minute, (nbr.val - 1) * 30, @date) as period_start, 
   dateadd(minute, (nbr.val    ) * 30, @date) as period_end 
   from numbers as nbr)
select pers.period_start, @period as Period, src.trunk, count(src.trunk) as 'all' 
/*, count(Simultaneous) as 'sim' */ 
--from periods as pers left  join @raw_data as src 
from periods as pers inner join @raw_data as src 
on src.StartDateTime >= pers.period_start and src.StartDateTime < pers.period_end
group by src.trunk, pers.period_start, pers.period_end
order by src.trunk
;

GatewayName	StartDateTime	DisconnectDateTime	ConnectionDuration	Trunk
GW1	2021-02-24 20:01:00.0000000	2021-02-24 20:05:30.0000000	270000	T1
GW1	2021-02-24 20:05:20.6000000	2021-02-24 20:07:50.1000000	149500	T1
GW1	2021-02-24 20:04:50.0000000	2021-02-24 20:08:24.0000000	214000	T1
GW1	2021-02-24 20:15:50.0000000	2021-02-24 20:17:00.0000000	70000	T1
GW1	2021-02-24 20:20:50.0000000	2021-02-24 20:21:00.0000000	10000	T1
GW1	2021-02-24 20:20:59.0000000	2021-02-24 20:24:00.0000000	181000	T1
GW1	2021-02-24 20:25:00.0000000	2021-02-24 20:28:30.0000000	210000	T1
GW2	2021-02-24 20:41:49.0000000	2021-02-24 20:43:24.0000000	95000	T2
GW3	2021-02-24 22:46:54.2000000	2021-02-24 22:48:25.2000000	91000	T1
GW99	2021-02-24 22:47:25.1000000	2021-02-24 22:47:54.4000000	29300	T2

(No column name)	val	period_start	period_end
2021-02-24 00:00:00	1	2021-02-24 00:00:00.0000000	2021-02-24 00:30:00.0000000
2021-02-24 00:00:00	2	2021-02-24 00:30:00.0000000	2021-02-24 01:00:00.0000000
2021-02-24 00:00:00	3	2021-02-24 01:00:00.0000000	2021-02-24 01:30:00.0000000
2021-02-24 00:00:00	4	2021-02-24 01:30:00.0000000	2021-02-24 02:00:00.0000000
2021-02-24 00:00:00	5	2021-02-24 02:00:00.0000000	2021-02-24 02:30:00.0000000
2021-02-24 00:00:00	6	2021-02-24 02:30:00.0000000	2021-02-24 03:00:00.0000000
2021-02-24 00:00:00	7	2021-02-24 03:00:00.0000000	2021-02-24 03:30:00.0000000
2021-02-24 00:00:00	8	2021-02-24 03:30:00.0000000	2021-02-24 04:00:00.0000000
2021-02-24 00:00:00	9	2021-02-24 04:00:00.0000000	2021-02-24 04:30:00.0000000
2021-02-24 00:00:00	10	2021-02-24 04:30:00.0000000	2021-02-24 05:00:00.0000000
2021-02-24 00:00:00	11	2021-02-24 05:00:00.0000000	2021-02-24 05:30:00.0000000
2021-02-24 00:00:00	12	2021-02-24 05:30:00.0000000	2021-02-24 06:00:00.0000000
2021-02-24 00:00:00	13	2021-02-24 06:00:00.0000000	2021-02-24 06:30:00.0000000
2021-02-24 00:00:00	14	2021-02-24 06:30:00.0000000	2021-02-24 07:00:00.0000000
2021-02-24 00:00:00	15	2021-02-24 07:00:00.0000000	2021-02-24 07:30:00.0000000
2021-02-24 00:00:00	16	2021-02-24 07:30:00.0000000	2021-02-24 08:00:00.0000000
2021-02-24 00:00:00	17	2021-02-24 08:00:00.0000000	2021-02-24 08:30:00.0000000
2021-02-24 00:00:00	18	2021-02-24 08:30:00.0000000	2021-02-24 09:00:00.0000000
2021-02-24 00:00:00	19	2021-02-24 09:00:00.0000000	2021-02-24 09:30:00.0000000
2021-02-24 00:00:00	20	2021-02-24 09:30:00.0000000	2021-02-24 10:00:00.0000000
2021-02-24 00:00:00	21	2021-02-24 10:00:00.0000000	2021-02-24 10:30:00.0000000
2021-02-24 00:00:00	22	2021-02-24 10:30:00.0000000	2021-02-24 11:00:00.0000000
2021-02-24 00:00:00	23	2021-02-24 11:00:00.0000000	2021-02-24 11:30:00.0000000
2021-02-24 00:00:00	24	2021-02-24 11:30:00.0000000	2021-02-24 12:00:00.0000000
2021-02-24 00:00:00	25	2021-02-24 12:00:00.0000000	2021-02-24 12:30:00.0000000
2021-02-24 00:00:00	26	2021-02-24 12:30:00.0000000	2021-02-24 13:00:00.0000000
2021-02-24 00:00:00	27	2021-02-24 13:00:00.0000000	2021-02-24 13:30:00.0000000
2021-02-24 00:00:00	28	2021-02-24 13:30:00.0000000	2021-02-24 14:00:00.0000000
2021-02-24 00:00:00	29	2021-02-24 14:00:00.0000000	2021-02-24 14:30:00.0000000
2021-02-24 00:00:00	30	2021-02-24 14:30:00.0000000	2021-02-24 15:00:00.0000000
2021-02-24 00:00:00	31	2021-02-24 15:00:00.0000000	2021-02-24 15:30:00.0000000
2021-02-24 00:00:00	32	2021-02-24 15:30:00.0000000	2021-02-24 16:00:00.0000000
2021-02-24 00:00:00	33	2021-02-24 16:00:00.0000000	2021-02-24 16:30:00.0000000
2021-02-24 00:00:00	34	2021-02-24 16:30:00.0000000	2021-02-24 17:00:00.0000000
2021-02-24 00:00:00	35	2021-02-24 17:00:00.0000000	2021-02-24 17:30:00.0000000
2021-02-24 00:00:00	36	2021-02-24 17:30:00.0000000	2021-02-24 18:00:00.0000000
2021-02-24 00:00:00	37	2021-02-24 18:00:00.0000000	2021-02-24 18:30:00.0000000
2021-02-24 00:00:00	38	2021-02-24 18:30:00.0000000	2021-02-24 19:00:00.0000000
2021-02-24 00:00:00	39	2021-02-24 19:00:00.0000000	2021-02-24 19:30:00.0000000
2021-02-24 00:00:00	40	2021-02-24 19:30:00.0000000	2021-02-24 20:00:00.0000000
2021-02-24 00:00:00	41	2021-02-24 20:00:00.0000000	2021-02-24 20:30:00.0000000
2021-02-24 00:00:00	42	2021-02-24 20:30:00.0000000	2021-02-24 21:00:00.0000000
2021-02-24 00:00:00	43	2021-02-24 21:00:00.0000000	2021-02-24 21:30:00.0000000
2021-02-24 00:00:00	44	2021-02-24 21:30:00.0000000	2021-02-24 22:00:00.0000000
2021-02-24 00:00:00	45	2021-02-24 22:00:00.0000000	2021-02-24 22:30:00.0000000
2021-02-24 00:00:00	46	2021-02-24 22:30:00.0000000	2021-02-24 23:00:00.0000000
2021-02-24 00:00:00	47	2021-02-24 23:00:00.0000000	2021-02-24 23:30:00.0000000
2021-02-24 00:00:00	48	2021-02-24 23:30:00.0000000	2021-02-25 00:00:00.0000000

period_start	Period	trunk	all
2021-02-24 20:00:00.0000000	30	T1	7
2021-02-24 22:30:00.0000000	30	T1	1
2021-02-24 20:30:00.0000000	30	T2	1
2021-02-24 22:30:00.0000000	30	T2	1