I am in support of this rule, for the most part. However, there are two clarifications that need to be made to make this rock solid.
The first, being the one Damian identified, being that there needs to be some sort of time limit on it, and a reasonable one. Just becase you attack another gang, does not mean you should have to stay on for three hours afterwards, just so the other gang can get enough people to retaliate. The time limit should be maybe 30 minutes at most in my opinion, that is more than enough time for the opposing gang to retaliate, or for the police to investigate.
Secondly, does this rule apply to the gang who attacked and the gang who was attacked? I can see that this rule, if it applies to the gang that was attacked, will probably cause more trouble between gangs, and more rule-breaking would occur. Why? Because, if you are just attacked out of the blue, or for some stupid reason, you'll want to go back and retaliate. So you'll probably be using less roleplay and it will be more like a big deathmatch between gangs. Therefore, I believe this rule should only apply to the attackers in a gang shootout situation, and not the persons who were attacked.
Also, I forget who said it, but they pointed out: That after being attacked, if they die, they like to log out right after getting out of the hospital. This might not be such a bad idea, because it makes roleplay more believeable. In real life, after being in a gang war, if you either die or get injured, you aren't right back out there two or three minutes later with a gun looking for your attackers.