The Full Form of GPDK is‍ Generic Process Design Kit.

A process design kit is a set of files used within the semiconductor industry to model a fabrication process for the design tools used to design an integrated circuit. The PDK is created by the foundry defining a certain technology variation for their processes. It is then passed to their customers to use in the design process. The customers may enhance the PDK, tailoring it to their specific design styles and markets. The designers use the PDK to design, simulate, draw and verify the design before handing the design back to the foundry to produce chips. The data in the PDK is specific to the foundry’s process variation and is chosen early in the design process, influenced by the market requirements for the chip. An accurate PDK will increase the chances of first-pass successful silicon.

They essentially allow you to design using a “generic” 0… A PDK is a “Process Design Kit” – it’s a set of libraries and associated data (model files, physical verification rule files, control files for various tools) to allow you to design in a particular technology.