It's somehow confusing to start a discussion of homemade PCB problems inside a thread about gerber file requirements for professional PCB makers.
The clear convention is to plot all gerber files non-mirrored, as viewn from the PCB top side. Presumed the films produced from the gerber files are contact copied to the board, the data for all layers with copper side up (top, each topside of multilayer substrates) have to be mirrored before being send to the photo plotter. The PCB manufacturer does this for you, don't need to care for.
Similar mirror actions have to be performed in DIY PCB processes, depending on the used technique. Basic point is to do all copy actions with direct layer contact. Respectively the top print has to be mirrored in direct photo or thermo transfer.