Compile OpenGL Program (Missing GL/gl.h)

I'd guess you don't have whatever dev package(s) provide gl.h. apt-file is the command line tool to use.

First, make sure it's up to date:

apt-file update

Now you can write a search to answer "what package provides the gl.h file?"

apt-file search "gl.h"

I don't have an ubuntu box just now to check, but I'd guess the result will be something like mesa-dev or opengl-dev.

Once you know what package provides "gl.h", (let's pretend the result from apt-file is "libmesa-dev"), install it:

apt-get install libmesa-dev

More info on the apt family of commands: https://help.ubuntu.com/community/AptGet/Howto

For GL/gl.h use:

sudo apt install mesa-common-dev

You may also find these helpful, for, say, GL/glu.h:

sudo apt install libglu1-mesa-dev freeglut3-dev

You can also build glproto (link), libdrm (link), and then mesa (link) from source. While many other packages may contain this header file, it's important to go to the source. This will reduce the memory footprint of your installations and ensure you have the latest and/or greatest version.