Air Carrier Captain reported being notified by a Flight Attendant of a 'gas station' odor in the cabin during cruise. Flight crew continued to destination where an AML discrepancy and fumes reports were submitted.

Narrative

While at cruise at flight Level 360; we received a call from the Flight Attendant notifying us that they had a 'gas station' like odor coming from the back of the aircraft. They said it was stronger in the back then in the front. I asked them if they observed any other characteristics; such as smoke or fumes; and if they were currently having any physiological effects. They said no to both questions. I said to please keep me informed if anything should change. This occurred within approximately twenty miles of the top of descent. All engine and flight instrumentation was normal. The First Officer and I did not smell the odor at this time; but per FOM guidance; continued to monitor the situation. We requested an early descent and requested our planned approach into ZZZ; the RNP [Runway] XX as soon as possible. During the descent I called the Flight Attendants back to ask if the situation had changed; and they said all was the same except that the odor seemed to be getting stronger but that it was hard to tell. We (pilots) still could not smell it at this time; and we continued with the approach and decided it was best to get the aircraft on the ground as soon as possible. The rest of the approach was uneventful. We landed straight in on Runway XX. After landing; decelerating through approximately 60 knots; the Master Caution and associated ELEC amber light on the overhead panel illuminated. At this point in time we still did not smell the odor. After parking the jet and shutting down the aircraft; the First Officer and I then almost immediately smelled an out of place smell. We immediately opened the windows; conducted the shutdown checklist and then I proceeded to the cabin to investigate further. After opening the flight deck door; I was met with a strong order smell from the cabin.The Flight Attendants handed me a cabin air quality reporting form. In that form; they observed the odor coming from the forward/mid/aft cabin and galleys. They described the smell as exhaust & fuel in nature; with a moderate intensity. It was at cruise flight and was noted at XA:10. Symptoms reported were a slight headache and sore throat; reported by the B and C positioned Flight Attendants. The physiological effects noted were not previously known until I physically received the report.Maintenance met the aircraft; and I immediately began to brief them. After a quick detailed explanation; the mechanic advised that everyone needed to be off the plane. The rear doors; along with the R1 door were opened for ventilation. The Flight Attendants deplaned after all the passengers; and we as crew debriefed the event in the jetway for approximately 15 minutes. While we followed FOM guidance closely; I do feel that I could have been more detailed on my maintenance logbook right up. I did speak with Maintenance prior to the write up; but given the vague nature of this situation; more detail would have been helpful.Based on the illumination of the ELEC light; and after consultation with maintenance; it appears the cause of the odor was in issue within the DC electrical or standby power system; though further investigation will be needed. This biggest lesson learned for me is just how insidious both electrical an odor events can be. Though I felt I worked through the issue properly; I do feel I could have been more prompt in my actions. In the future I will ask more pointed questions in a more prompt manner; which will allow us that much more time to take the appropriate actions. And while I feel that declaring an emergency was not needed in this case; it was clear to me that it could have quickly developed into one given different circumstances.