Everything by donttouchthederp (link: http://www.asianfanfics.com/profile/view/57233)
^ She is my absolute favourite author on aff (and I'd suggest everyone to read her fanfics, even though you're not a g-dragon fan)
Also Cruel Fate by almostblue (link: http://www.asianfanfics.com/story/view/41137/cruel-fate-bigbang-gdragon-jiyong-vampire)
^ One of my absolute favorite authors on aff as well

Have fun reading~

The best is Mob for Rent!

Read Redemption by xoxkwonlover :) not sure if there's enough "gang aspect" for you but I loved that fic!