I've tried setting a custom shortcut for gnome-system-monitor on SHIFT+CTRL+ESCAPE because that's what I use on windows and I want it on ubuntu too. But it doesn't work. I tried setting the command, from settings-keyboard-shortcuts, as simply "gnome-system-monitor", then as "/usr/bin/gnome-system-monitor", and then tried setting it from compiz manager under commands. Nothing worked. My other shortcuts work (for example I have a shortcut of alt+win+f4 for poweroff). I have ubuntu 16.04 64-bit.