Gmail was usable on Windows Phone in the built-in mail app, YouTube had some okay third party apps, but in my opinion what really killed Windows Phone was not having Google Maps. People underestimate how critical this app is. Why do you think Apple created their own mapping app out of nowhere? They were terrified that Google could turn off the switch and cripple iOS overnight after watching Windows Phone wither on the vine.

There were additional things that caused Windows Phone to fail, but I really feel Google Maps was the first nail in the coffin.