CMake unable to find GCC

Whenever I try to run anything on CLion i'm getting the following error :

CMake Error at CMakeLists.txt:2 (project):
No CMAKE_CXX_COMPILER could be found.

Tell CMake where to find the compiler by setting either the environment
variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
to the compiler, or to the compiler name if it is in the PATH.

So CMake can't find gcc (which I have because I checked on terminal). My problem is I have no idea how to tell CMake where gcc is. I did check numerous stackoverflow but it's mainly about visual studio and I'm so noob I don't understand where I need to run the lines.