I had some weird error when trying to install the widescreen patch, seems the GOG installer installs some WeiDu-Stuff (DirectDraw-Fix), I had to delete this file (setup-directdraw.exe or so), before installing the widescreen patch, otherwise the installation of the widescreen patch did not work (Win7x64)
This is because the WeiDu installer for the current Widescreen mod, (currently v3.05) is a bit older. When you run the installer, it checks for other WeiDu mods already installed on the game and it doesn't like newer versions of Weidu. WeiDu is actually a mod installer package system developed for Infinity Engine games that keeps track of your installed mods to keep them compatible with each other. Overall, WeiDu is great, but it does have a couple of quirks, like the one you mentioned. This will likely not be a problem, as soon as the next version of Widescreen mod comes out. In the mean time, you can work around it, jut by making sure you temporarily move other WeiDu mods out of the folder (like the DirectDraw-Fix) and install the Widescreen mod before any other mods (which should be done anyway).