Package gsl was not found in the pkg-config search path.

This error message suggests that the `pkg-config` tool could not find the `gsl` package in its search path.

To resolve this issue, you can try installing the `libgsl-dev` package using your system's package manager. For example, on Ubuntu or Debian, you can run:

sudo apt-get install libgsl-dev

If you are using a different operating system, you will need to consult its documentation or community for guidance on how to install the `gsl` package and add it to `pkg-config`'s search path.