G_ mail mobile app was launched way back in 2011 but very few people got their hands to use the instant app. Because many of them were using Symbian phone rather than Android.
This app doesn’t let you to open your account by always putting that email with password and it has benifit of synchronisation mease if any new mail arriver it will notify you.
The user interface is very simple so anyine can use this app and its completely free on playstore as well as iStore. I think this is the best app for getting things done instantly.