To advance past the gorge, you will need three items.
1. The ramp (acquired while getting The Spirit of Ricky Myran achievement).
2. A boost canister. This is acquired during a fight with a dark haired biker on the Mine Road. However before you get it, you will need to acquire the chain from another biker fight. If you try to use any other weapon, he will activate his booster and leave the fight. Once you have the chain, equip it for the fight, and immediately attack the biker when you can. You must get him before he takes off with the booster. When you down him the booster will automatically be installed. If you miss him, just continue driving down the Mine Road until he appears again.
3. A fan part. This is acquired after Nestor and Bolus crash their car (see Fertilized achievement). Drive down Highway 9 until you have an option to exit at the car crash site. Use the tire iron on the car toward the back end. Ben will open a panel. Remove the fan. Then use the fan on the bike. It automatically installs.
Once all three items are in place, drive to the gorge. While on the bike, move your cursor towards the gorge until it becomes a drive arrow. If Ben says that he has modified his bike enough (and the ramp is in place), a cut scene will begin and the achievement pops. If you are missing of the items, he will comment that the bridge is out.