Sorry, but that's a load of bullshit. LA is infact the gang capital of the nation, so I have absolutely no idea where you got that information from.
There are over 37,000 known gang members in Los Angeles within the city of Los Angeles and LA county. Although that is considerably lower than the 65,000 of 1997, it is still a significant number.
Source
I'm sure we will see gangs in this game, it's almost certain. I shared my opinions on some other topic somewhere before, so I won't bother again. But I doubt the main storyline will revolve around gang culture, so while they'll be there they won't play a major role like how it was in SA. Maybe for a EoLC type DLC, but not the main game.