How to use find_package in CMake? (Example: GMP library) - cmake

Question

I'm trying to use find_package to include libraries in CMake.
This question talks about how to tell CMake to link to the GMP library (external). I am trying to follow the steps of the answer there but do not have any of the <name>Config.cmake or <name>-config.cmake files, as mentioned by some of the comments, which appears to be the default. The answer does not mention any solution for when you don't know how to get/find these files. The comments to that answer link to an old website (external) with a lot of broken links, that describes a list of Load Modules. It's unclear to me where these modules come from and how to get them.
According to the official CMake documentation (external), if the configuration files are not found, find_package falls back from "Module Mode" to "Config Mode". I don't understand what this means and in what cases this would be relevant, especially since the documentation discourages reading about "Config Mode".
The documentation says that
The file is first searched in the CMAKE_MODULE_PATH, then among the Find Modules provided by the CMake installation.
I am still confused about whether these configuration files are supposed to come with CMake or with the library in question and where they are supposed to be located. Probably both are possible but how does one know in a specific case?
Example code, trying to follow modern best practices:
# CMakeLists.txt (not working)
cmake_minimum_required(VERSION 3.2) # I have no idea what version I actually need
project (GMP_demo_project)
# Enable C++17 standard
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(GMP REQUIRED)
# Create the executable from sources
add_executable(GMP_demo GMP_demo.cpp)
target_link_libraries(GMP_demo gmp gmpxx)
The code outputs an error message along the lines of
CMake Error at CMakeLists.txt:10 (find_package):
By not providing "FindGMP.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "GMP", but
CMake did not find one.
Could not find a package configuration file provided by "GMP" with any of
the following names:
GMPConfig.cmake
gmp-config.cmake
Add the installation prefix of "GMP" to CMAKE_PREFIX_PATH or set "GMP_DIR"
to a directory containing one of the above files. If "GMP" provides a
separate development package or SDK, be sure it has been installed.
Question: How does one, in general, obtain and organize these configuration files (CMake Load Modules)? How can one expect another user to have these files on his system? My question is intended to be general and only use GMP as an example (although I am in fact interested in being able to use it).
Just as an aside, I can compile, link and execute my demo code just fine using gcc GMP_demo.cpp -lstdc++ -lgmp after having installed GMP as suggested by the library documentation. The problem is just getting CMake to do it. I can also just give CMake the absolute path of the library, which would of course be much easier but not portable (assuming one can get find_package to actually work and be portable with reasonable amounts of work).

Answer 1

How does one, in general, obtain and organize these configuration files (CMake Load Modules)?
Broadly speaking, there are three buckets these fall into:
Files provided directly by the package. This is the ideal solution, and would be what CMake calls Config mode. There would be a file called GMPConfig.cmake which cmake could find by searching preconfigured paths, or by providing a specific path at configuration time (cmake -DGMP_Dir=/path/to/GMP/install/root). The advantages of this approach are that generation of GMPConfig.cmake is mostly automatic, and the libraries can include things like installation paths and compilation flags. The disadvantage is that the library develops have to actually go to the effort of leveraging modern CMake, and not everybody does this.
Files provided directly by CMake. For common packages (e.g., boost) CMake ships FindXXX.cmake files that search well-known paths and take care of this for you. These work identically to the above from an end-user perspective, but which Find modules are available depends on the version of CMake you have installed.
Files provided by some random person that are copy/pasted into projects. How these works depends on the person who wrote it, so you'll have to read their documentation. Use your favorite search engine and try to find FindGMP.cmake, then drop it in a module folder somewhere and update CMAKE_MODULE_PATH appropriately.
How can one expect another user to have these files on his system?
It's your job to install whatever dependencies a package requires. Anything using modern CMake (bullet 1 listed above) should install the XXXConfig.cmake file as part of its installation. If a library is built by something other than CMake, you'd have to either hope for bullet #2, or find/write your own FindXXX.cmake file (bullet #3).
For your specific case, you might be better off with find_library, since your sample compilation line looks like it just needs to link.