How do I use it on Ubuntu?

Last verified against genymotion.com:

On Ubuntu, Genymotion Desktop is meant to run with the built-in QEMU hypervisor, so you do not need to install extra software. It is officially supported on Ubuntu LTS, and Wayland is recommended because X11 may crash in Ubuntu. Get Started with Genymotion Desktop Get Started with Genymotion Desktop

Use the Linux installation guide, then launch Genymotion Desktop normally after installing it. If you choose VirtualBox on Linux, it is legacy only and not recommended.