" Guake is a top-down terminal for Gnome, and is highly inspirated by the famous terminal used in Quake .
You can instantaneously show and hide your terminal with a single key stroke, execute a command, and then go back to your previous task without breaking your workflow. "
It's maintained on GitHub, so you might be able to manually install it or find a package for it in your package manager. At least on Arch you can install gnome-shell-extension-ddterm-git from the AUR.