Assuming you have a Gnoppix Core/PRO 23.15 installed and ~25GB space left currently KDE+XFCE and GNOME are supported. see requirements
There are 2 Ways to start (initial install) the application
Methode 1.)
Goto: Menu → Internet → Gnoppix diffusion
As a first time usage, the Gnoppix Diffusion automatically download all needed files from the internet.
make sure you have a working internet connection. Around 10GB will be transferred, this can take, depend on your internet connection serval minutes to hours or days.
Methode 2.)
Open a Terminal and type: