You have to include the header file(s) where these functions and types are declared:
#include <glad/glad.h>
The path depends on where the file has been installed. The above assumes that it is the
glad sub directory of the directory where your source file is located. If it is somewhere else you have to add the path to the list of include directories searched by your compiler (project settings when using an IDE, shell variable or compiler command line option when using a make file or compiling manually on the command line).
[EDIT]
A quick web research revealed that
glad.h must be included before
glfw3.h:
#include <glad/glad.h>
#include <GLFW/glfw3.h>
[/EDIT]