The most important thing the gangs can provide is security. Safe houses in gang territory, lookouts, the willingness to start a quick riot is a raid is attempted on a safe house so as to allow terrorists to escape, etc, etc.
While gang members would not be suicide bombers, they could work as agents all over the place -- in meat processing plants, as maintenence men in buildings, etc. They can quietly go anywhere with little suspicion, gathering info, and maybe doing operations for the terrorists. Poison in the bacon, anyone? Perhaps the fuss with the poisoned pet food was a message?
You are absolutely right. Gangs are so much part of American societal weave that they become a horrrific "enemy within". And unlike the terrorists an "invisible" part of our danger.
I have a sneaky feeling that after some rather horrible event, they might well turn on the terrorists. Money only buys a cerain amount of loyalty. Specially if a gang member or their family is killed in a terrorist suicide attack.
While there are some ruthless elements like the "13" gang most of the others will back away or even oppose the terrorists at some point in time.