Not a mechanic

Okay I have a 06 Nissan Maxima was driving it I don't know where it did a misfire and sputtered into AutoZone's parking lot would not start after just had a new cat put on change all six plugs and all 6 coil packs there's gas in it fuel pump is pumping had AutoZone scan the car they told me misfire and bad head gasket which I really don't believe brought it to another shop they said the timing was off and there was no spark which I kind of feel like they just didn't want to deal with the car does anyone know any other possible reason as to why it doesn't want to start really would love to fix the car without sending it to the dealer