You don't need to unlock, if screen is on it will work.The beauty of google pay is you don't need to open anything! No apps or anything! This is why I moved from Samsung pay.
As long as your phone is unlocked, it will work. On any screen.
So all you need to do, is take out of your pocket. Unlock, then hold to the terminal to pay. You dont need to open the app, while paying...or ever really!