How to use gtk with cmake under Windows platform?

Considering you are using GTK 2.20.0 (i.e. version> = 2), you should use GTK2 instead of GTK. But if FindGTK2 has the same problem, you can use find_path to find the header files and you can use find_library to find the associated library files. By combining them, you can create the GTK2_FOUND, GTK2_LIBRARIES and GTK2_INCLUDE_DIRS symbols that it should produce. You can find my FindUnitTestPlusPlus.cmakethe file is a little useful; it uses "FindPackageHandleStandardArgs" which takes care of the verbose details that you won't fail if the library isn't listed there and the REQUIRED flag is specified. Unfortunately, there really isn't a lot of documentation on creating custom FindXYZ.cmake modules; however, I think you should be able to hack something along with the existing modules.

Another option is to contact the developer of that particular module. A list of CMake module lookup helpers can be found here. Philip Lowman is a popular guy for the FindGTK2 module.