I have no idea. (I'm sorta out of the rapper loop) I just know I have seen a lot of pics of Joran & Friends using that sign. Could be just a teenage thing.
There's been some discussion about the finger gestures here on FR in the past and the consensus was that it apes the nonsense you see on stage, etc. Anti-establishment at the very least. Extremely interesting when you connect that with Joran and his pals' websites' contents, etc. and the dreadful attacks on women tourists in Aruba. The attacks, rape and robbery, occur the night before tourists leave, so they're reluctant to remain on the island to press charges. This would be quite a feather in Joran's cap, if he managed to carry it off. Perhaps that was what he and the Kalpoe's were up to, but it went horribly wrong.
My guess is it is a derivation of the V sign in the UK, palm facing out is peace or victory, but palm facing in is F*** Off, equivilant to American middle finger. I do not think it is gang related, however, it does make you look extremely cool when photographed at the pub.