Try modified code below.
SELECT
Gang,
[1] AS Januar,
[2] AS Februar,
[3] AS März,
[4] AS April,
[5] AS Mai,
[6] AS Juni,
[7] AS Juli,
[8] AS August,
[9] AS September,
[10] AS Oktober,
[11] AS November,
[12] AS Dezember
FROM
(
SELECT MONTH(Monat) my_month, [Zu-Abgang] As Gang, COUNT(DISTINCT Projects) ProjectCount
FROM [dbo].[tbl1]
GROUP BY MONTH(Monat), [Zu-Abgang]
) AS t
PIVOT (
SUM(ProjectCount)
FOR my_month IN([1], [2], [3], [4], [5],[6],[7],[8],[9],[10],[11],[12])
) as p