GtP stands for Gouda Tobacco Pipes. The pipes are made in Gouda, The Netherlands, by a man named Adriaan van der Want. He is the last in a long line of Gouda clay pipe manufacturers.
Nice example, courtesy Flemming Lyngholt Hansen
The pipes are very pretty to look at and they come in different sizes, shapes and colours. I have 3 GTP pipes, never bought one though. The pipes give a reasonably cool smoke especially compared to the traditional long Gouda clay pipes. However they tend to get hot and because they have a double siding you can get some trouble with [moisture]. As far as clay pipes go, the GTP pipes are rather good.
From the following A.S.P. thread: https://groups.google.com/forum/m/#!msg/alt.smokers.pipes/DlI7WFNYVeM/0JVc3Xsy_oIJ