Does anyone know if the GCM native notification prompt you see on Browsers such as Chrome, can be replaced or adjusted in any way so that we show a more presentable popup instead?
Here is a native prompt example:
No, you cannot customize it.
The only option would be do draw your custom dialog with HTML / CSS, then when someone clicks subscribe display the default browser dialog: I've already seen it on some websites, but I strongly recommend against it (because of the bad UX it provides).