Luca ripped off Ray after the previous mission, so Ray wants you to head over to Castle Garden City and get his stuff back.
Chase That Traitor
Grab a vehicle and make your way to the destination using your GPS. After the cutscene, Luca will try to escape so he can tell Ray it was you who stole the diamonds. Chase after him in the Banshee and shoot the car, but don't blow it up. All of the diamonds will be lost if Luca dies. Try to kill the guy hanging out of the window shooting at you. Once he's dead, it'll be a simple matter of keeping up with the car.
It'll eventually crash in Middle Park, then you'll be asked to take out the gang. Use the near-by hot dog vendor if you need some health, then move in on foot, using cover and kill the three men. Luca will be hiding in the toilets, so move into them and you'll be told to shoot the doors open until you find him. Once you've got him, move in and execute him.
Niko will then call Ray and tell him that he killed the gang and he's got the diamonds. He asks Ray to meet him on the bridge, so you need to head across the park and meet Ray to give him the diamonds.
- There is a fair amount of cover available in middle park, and with a medium range weapon such as an SMG or Assault Rifle, you shouldn't have much trouble taking out the remaining gang members.