How can I jump across the bridge?

If I remember correctly you need to get out on the middle span and get it to tilt down towards this span, like a seesaw - it's got a bunch of heavy crap tilting it the other way that can be knocked off with the gravity gun.

It's been a few years, though. That might be total malarkey.

If I'm remembering this part correctly, you should be able to cross to the other side of the bridge on foot. Rearrange the cars with the Gravity Gun so that the weight causes the bridge to tip down, then you can drive across.