Some spoilers for 30 Nights follow
My suggestion is to not throw them basic grunts like the book suggests, if your players are looking for more engaging or challenging combat. You are hampered somewhat by the setting. There's no power, so tech attacks are mostly off the table. There are very few vehicles, so threats like Law Enforcement don't have teeth. The players often can bring their full force to bear on whatever they're up against without repercussion.
30 Nights calls for a lot of gangers. Here's what you can do to make gangers more dangerous:
1. Give them a gimmick. Grenades, fire, toxins, animals, social consequences. These are all things that make low level opponents more tricky to navigate.
2. Numbers. The basic TTRPG challenge increaser. Bring more of them. It's not clever, but it works. When I started 30 Nights, I emphasized that the Ares pullout caused lots of unemployment in the city. This is why gangs are able to bolster their numbers so much in the blackout. Be careful using the Grunt Group rules. In most cases, this reduces the threat of large groups. Only use it as a time saver.
3. Add tech and magic. When you add these dimensions to a group, it engages more of the party and makes them think tactically. It's not easy to add tech in this setting. I wouldn't do it until the second half of the campaign. If you and your group thinks that adding magic to gangs is unrealistic, I'd suggest doing it within the context of a gimmick. You have a fire themed gang, you include a mage that can call fire spirits - and you HIDE them because they're target #1 for your players. If your players wonder why there seems to be a lot of magic in street gangs, tie it in to the Black Lodge.
4. Level up old threats to keep them relevant. In my campaign, there were two instances of threats that came back later in the campaign, but tougher. A fire themed gang I made up called "Red Promise" came back as the "Black Promise" when I combined them with the shadow spirit possessed corpses from Night 4. They used chilling tactics such as crashing a remotely driven pickup truck full of zombies into the players. Another gang that was violence-prone leveled up when they gained a benefactor, Wuxing. Wuxing used them as foot soldier proxies against Peregrine and the runners, who they saw as being Saeder-Krupp's proxies. The rivalry between these two corps is important in the second half of the campaign. A couple dozen brand new (Wuxing rebranded) AK-97s with explosive ammo can do a lot of damage, and these guys loved to do damage.