Official Patches
GPL 2004 demo
This provides a useful starting point for updating GPL, as it already has the CPU speedfix patch and the D3D patch. Because it does not inlcude the original tracks it does not include the updated files included in version 1.2 for the original tracks.
GPL version 1.2
The first thing to do is to apply the official Papyrus patch to update the game to version 1.2. This and all other "official" patches can be found at
N.B. When I applied the patch, some of it failed because all GPL's track files are read only. Change the properties of these files to make sure your patching goes smoothly.
CPU Speed Fix patch
This patch is designed to solve problems with processors which run at over 1 GHz. It can be used with slower processors and has the advantage that the GPL CD is no longer required to run the game (UK version at least). A fresh installation of GPL Season will work with this patch, but if you have used GPL Season before applying the patch, GPL season will NOT work with the new patch. Stefan Magnusson's site GPL Addons has a solution in the How To - Other Stuff - GPL Season section. This patch includes the previous Connection Reliability Patch.
Direct 3D patch
Unless you have a Voodoo card, you will need the Direct 3D patch. Read the patch's readme files and create/modify your core.ini file accordingly. The new Direct 3D patch solves the problems with the previous patch, and I see no reason for using the Open GL patch.
Open GL patch
A modified version of this patch coupled with recent Nvidia drivers means OpenGL is now a viable alternative to D3D. Many report frame-rate improvements.