From IraqiNews.com:

A Kurdish official revealed on Tuesday evening that the ISIS organization had bombed large parts and tracts of the ancient Nineveh wall, indicating that such an act violates the right of human culture and heritage.
The media official of the Kurdistan Democratic Party (KDP) in Mosul, Saed Mimousine said in an interview for IraqiNews.com, “ISIS militants blew up today large parts and expanses of the archaeological wall of Nineveh in al-Tahrir neighborhood,” explaining that, “The terrorist group used explosives in the process of destroying the archaeological fence.”
Mimousine added, “The Wall of Nineveh is one of the most distinctive archaeological monuments in Iraq and the Middle East,” adding that, “The fence dates back to the Assyrian civilization.”

The full article includes a photo of the gate. A related article shows a photo of an explosion.

Nineveh is best known as one of the capitals of ancient Assyria. In the 8th century BC, Jonah visited the city and Sennacherib began construction on his “Palace Without a Rival.” Fortunately, many of the important artifacts were removed from the site in the 19th century and are now on display in the British Museum.

HT: Agade