You wouldn't have to completely collapse the bridge to have a big impact. It might be possible to do enough damage to the bridge that it would be closed for months. That would have a big impact on traffic in the Bay area.

The main support cables on the Golden Gate Bridge are massive, something like 30 inches in diameter if memory serves. The vertical support wires are quite a bit thinner and might be broken by a big plane or something. You're right that it wouldn't be easy for an inexperienced pilot to pull that off but it's far from impossible. The World Trade Center towers were pretty narrow targets yet both were hit.

A large truck could carry a lot of explosives. It could be even worse if they tamped the explosives with a lot of sandbags on top, directing the blast downwards. That would do the most damage to the roadbed. I don't know if that would be enough to blow a hole completely through the roadbed but it would likely close the bridge for quite some time.