Gmail I believe is one of the most old app of 21 century. I have only used this app almost a year. I was not known to this app before I used to check my emails on browser or my mobile inbuilt mail box. The things why I like this apps are
1) Its easy to use. Anyone can use it with minimum mobile user skill. It is a convince app.
2) It is very fast. FOR any app it is much more important to be fast and resposive. Gmail app is one of them.
3) It is directly connected to google drive. It is not forcefully connected but it is a mobile friendly app.
These are the three things I like about this app but only thing I dont like about this app is too many notification.