With every reservation about the correctness, here's what I think:
1) Network Layer
- Gateways are an IP construct.
2a) Physical Layer
- Just a reasonable guess. It makes sense to me that the serial encapsulation would fail, if connectivity at the physical layer failed.
2b) Datalink Layer
- To my understanding authentication mechanisms in PPP work at the datalink layer, since they are part of the LCP (Link Control Protocol).
Again I haven't verified the answers, they may be wrong, especially since the questions are theoritical. Perhaps someone else can say for sure.