GPA determines what firms you can get into, generally speaking.
Although my school doesn't release class rank until after graduation, its pretty easy for potential employers to figure out your approximate class rank based on the curve of your school. At Michigan, the 1L curve is about 3.25, with 10% below a 3.0, and so forth. So the prestigious firms might only accept people with a 3.6, the normal run-of-the-mill Big Laws might take people with at least a 3.1, and if you are 2.9 or lower (bottom 5 ish %) you might be SOS even at a T14 for market rate Big Law at least.
Those are just numbers I'm pulling out of my arse, but I think they would hold up as pretty representative generally speaking.
At a school like UCLA, I would think you need to be in the top half of the "curve" to get Big Law, and maybe more like top 1/3rd if you want to have a solid/near guarantee chance. Again, these are just water-cooler impressions I have and I don't possess any hard statistics to back this up.