Is it possible to launch GUI of headless VirtualBox' machine?
I just solved this problem by clicking the green arrow "Show" at the top of the VirtualBox GUI while my headless instance was already running.
I could see what the VM window would look like inside the "Preview" Pane on the VirtualBox GUI, and after clicking "Show", the VM window came up as though I had initially powered on the machine from the Virtualbox GUI.
Edit: When you close the VM window, it will give you the additional option to "Continue running in the background", so your headless instance can revert to being headless again.
You can attach to a headless VM by running VBoxSDL --startvm <VMname> --separate. If you run VBoxSDL with no args you can see other options you may be interested in.
You can click on the Show button to get the GUI of a headless VM in the VirtualBox Manager:
When you are done with using the GUI, you can detach it and your VM will go back headless.
I tested this on Windows 10 and it worked as of VirtualBox 5.1.22. Should be the same on Mac/Linux.