png to gbr conversion is probably searched by users looking for a way to convert a picture of PCB schematics to actually gerber file. This might be a bit tricky, but not impossible. It all depends if you can find a tool that can be used to transfer raster designs to actual PCB formats.
It might also be possible that png to gbr conversion is related to making of brush collections in GIMP editor, in which case this is the tool you have to use for this.