For those looking for a solution that's easily installable on Ubuntu 18.04+, I found
qjoypad in the official Ubuntu repositories. Install it like this:
sudo apt install qjoypad
Then, it should be available in your launcher. It starts as a system tray icon, and you can launch the configurator by right clicking on it.
Personally, I really like the quick bind option that lets you press a key on a gamepad and then a key on your keyboard to update the mapping for that layout automatically. Note that it does indeed natively support multiple gamepads.
For me, I had some trouble with getting the right click to launch the configurator to work on the Unity 7.5 desktop - and the left-click menu doesn't appear to have an option for launching it. Thankfully, there's a workaround for this too. Simply launch
qjoypad like so:
....this will disable the system tray icon and instead launch a window with the icon in it instead - which works as expected.
This can be made to happen automatically by editing
/usr/share/applications/qjoypad.desktop. Find the line that starts with
Exec=, and append
--notray to the end of it (make sure that there's a space after
qjoypad at the end of the line and