All of the lights are on in my subaru outback 2011, check engine, brake light, oil light, tire pressure light. Thinking its a faulty gas cap or the battery, which do you think?

Hello, low battery voltage can definitely cause multiple warning lights to illuminate. Have your battery tested to see if it's weak or unable to sustain a necessary charge. Diagnostic testing is required to retrieve set diagnostic trouble codes that will indicate clues as to what is been recorded when lights are coming on. If there are any codes of substance not related to low voltage, it will take testing to pin point then cause.