1. GP Pipes = Galvanized Pipes (GP/GI Pipes) Often “GP pipe” refers to Galvanized Plain steel pipes (zinc-coated). Used for water supply, structural work, fencing, etc. 2. GP Pipes = General Purpose Pipes Sometimes industries call standard mild-steel pipes “GP pipes. 3. GP Pipes in Plumbing In some regions, “GP pipe” simply means non-pressure general-purpose plastic or steel piping.