Hi Simone, the errors that you are getting are compile errors, not linking errors. What happens is that Visual Studio needs to locate the header files like "gsl/gsl_mode.h" but apparently it cannot found them.
Normally you should make sure that you have actually installed these files and take note of where they are stored. These files are all provided by the GSL library. Once you are sure about where the header files are located you should go in the visual studio project and instruct application about the directory where to find them. You may follow the instructions here: https://stackoverflow.com/questions/4445418/how-to-add-additional-libraries-to-visual-studio-project I am reporting here the instructions: ------------ extract For Visual Studio you'll want to right click on your project in the solution explorer and then click on Properties. Next open Configuration Properties and then Linker. Now you want to add the folder you have the Allegro libraries in to Additional Library Directories, Linker -> Input you'll add the actual library files under Additional Dependencies. For the Header Files you'll also want to include their directories under C/C++ -> Additional Include Directories. If there is a dll have a copy of it in your main project folder, and done. ----------- end of the extract In your case it is the GSL library instead of "Allegro" but the same instructions applies. So you basically have to instruct the visual studio project (solution) - where the header files are located (gsl/gsl_mode.h etc, all the files terminating in .h). This is required for the compilation step. - where the library is located (.dll or .lib) so that it can be linked for the final step. For the GSL library you may have actually two libraries, libgsl.dll and libgslcblas.dll. Make sure that you have the libraries, where they are located and their names and add them into the project as explained above. Please note that if you downloaded the source code of the GSL library you will *not* have the libraries to link with. For this purpose you need to download the pre-compiled libraries for visual studio, for example using vcpkg. I hope this will be enough to get you started but I will be glad to assist you further! Kinds regards Francesco