As a database designer, I often create on database tables the primary key as identity autoincrement and sometimes I needed to find missing or deleted IDs thet interrupt the sequence of autoincrement number on the primary key.
To do this you can create a sequence from 1 (or 0 based on setted RESEED) to MAX ID available on the table and than make a LEFT JOIN of this sequence with start table.

The following snippet get the list of IDs of identity autoincrement column that interrupt the sequence of autoincrement values:

DECLARE @START_ID INT,  @MAXID INT

SET @START_ID = 1
SET @MAXID = (SELECT MAX(ID) FROM [TABLENAME_TO_CHECK])

DECLARE @SEQUENCE_TABLE TABLE (ID BIGINT)

--CREATE TABLE WITH ID FROM [@START_ID] TO [MAX TABLE ID]
WHILE @START_ID < @MAXID
	BEGIN
		INSERT INTO @SEQUENCE_TABLE VALUES(@START_ID)

		SET @START_ID = @START_ID + 1
	END

SELECT S.ID AS [MISSING ID] 
FROM @SEQUENCE_TABLE S 
		LEFT JOIN [TABLENAME_TO_CHECK] AS T ON  S.ID = T.ID
WHERE T.ID IS NULL