Not sure what kind of Gnome you are using, but in Gnome 3.26 as shipped with Fedora 27, in the Settings, Power options, you have three options what can happen, when you press the power button: "sleep" (the default), "shutdown" and "do nothing".

When you select the second option, "shutdown", and then press the power button, it will throw a dialog at you, that the machine will shutdown in 60 seconds (and will display countdown), together with "cancel", "reboot" and "shutdown" buttons.