Altium Designer from which .prjpcb files originate supports many different gerber formats, so gbp to prjpcb conversion essentially describes the import of one of the such files to the software, either to a new or existing project.
This is less of an actual file conversion and more about how you work with the software. You can find many guides and tutorials about how to import and work with various gerber files using Altium Designer.
Updated: October 27, 2023