Seriously, can you imagine a world where this could be possible? Schematics are representations of components that can handle high or low voltage which can be in discrete form, integrated circuit, surface mount, thru hole mount, signal, power, etc, etc... Layouts are drawn inside a board with specific borders geometry and fixation holes, upon specifications of components positioning so that there is no electrical mutual interference and mechanical overlay each other and with the sorrounding world. Without defining boundary conditions all you would have would be a soup of interconnected components...congrats.without you having to design it?
I'm assuming you have a JPEG that is of the PCB you want to turn into Gerber files. Circad )aka OmniGlyph loads in most of the popular graphics files. The idea is that you can just go with creating a Gerber but you can use the image of the PCB make it the top layer then place all the parts that are on the board and generate a netlist from the PCB which you can read in from a blank schematic file and it will give you schematic (A bit jumbled up but with the correct connections all direct to the pins of the parts)Is there a software that can convert a schematic like jpeg or any other drafting file into gerber file without you having to design it?