Let her vent and tell her not to apologize. That's what real friends are for no matter what even if they dont do the same for you. It just says what a kind hearted person you are.

I would say let her vent and let her know she doesnt have to apologize if shes saying things you arent comfortable with then maybe say something but otherwise be there for her

Depression already brings buckets of guilt along with it so it isn't surprising that you feel this way, however, we all need to unload every now and then and if she trusts you to tell you things that she is worried about, you shouldn't feel guilty and neither should she so I agree with Jillian. Tell her that's what friends are for and that she doesn't need to apologise for venting - you're more than happy to listen and help where you can :)

she's not venting about you, is she? because that's not cool.

Understand that depression makes you feel different ways but your gf is coming to you to vent if you feel bad for listen maybe you need to talk to her about it. She must want you to hear her venting. Most ppl after they vent do apologize for doing it.