Once on the far side of Stoner Cement Works you will see 2 bikers gun it on through. Give chase after them. Go immediately to the right. You need to take them down within 55 seconds for Gold. For Gold, you would want to Stun both of them using the stun gun.
The first to go will be left on their own while you deal with the second. Remember, for Gold, you need to zap them both. That or just get close enough for either Joe or Josef to Zap the illegals.
If you left the first target on the ground, then you will need to head back for him. Get them both in the car and the mission ends.
Grab the nearby bike that either of them were on and you can get back to civilization.