Well just to confuse things the gang attacks are at the following times for me:
Chumash Beach: 12:00 - 20:00
Brocker Park: 20:00 - 04:00
Mirror Park: 12:00 - 20:00
Maybe the times are set in stone like the other 56?
Here is a photo taken straight after I completed the Mirror Park one, shortly after it became available to play (early evening):
I didn't note what time it was in game, but it looks later than before 8pm.
In Typical Gamer's video, the time he finishes is at around 11pm.
However, on the 'East Vinewood' (Broker Park) one it is during the day in the video so probably 12:00 - 20:00
He does say 'both East Vinewood and Mirror Park seem to be activated around 11 o'clock at night, but try your luck at anytime' so it might be 'random' :0)