Walk up to the truck on the left side of Gridlock, walk up close to the edge of the left side of the truck and you should be standing right on the edge.
Now hold B and press A 3 times and release B. You should now be walking sideways, but you wont leave the truck (your screen will be shaking a lot if done correctly).
After 2 minutes and 30 seconds your screen will stop moving and you'll be running extremely fast sideways, still not leaving the truck. Now press B but don't hold it, roll backwards (down + A) and you'll go really high up and it should be black until you look down.
Now you should see Gridlock but its looking very small, now just roll towards the building you want to land on and you'll fall on the top of it.
Note: you can be shot when doing the skydive or when you have done it so be careful or you'll have to do it again.
PS: make sure you have a long range weapon or 2 with plenty of ammo, because when you run out, the only way to die is for you to be killed.