Product/Administrator's Guide has describes common IPsec log entries in one of the appendixes. Here's link to 5.8.3 online help that has same information:
However in this case it looks like you'll need to analyze the problem on peer GW side. The "No proposal chosen" message is just a generic error that NGFW shows when negotiation fails on the peer device side. Since peer device does not send exact details why it refused the proposal, NGFW will only show this generic message telling that negotiation has failed. So check the IPsec logs from peer device and make sure that IKE (phase 1) settings match in NGFW and peer GW configuration.