At the moment, they provide two very different views of email. Actually I guess you could say the old GMail client offers two different views itself, since it has both standard and priority inbox.
It's not clear that Inbox is the right way to look at email for the majority of users, and I'm saying that as a user/tester. I've had to drop back to the regular GMail app do do things a couple of times and Inbox has crashed twice on me.
Note that the GMail client did take on some of the look and feel such as the (+) widget on the bottom right.
The widget is the 'floating action button' in Material Design parlance. You're suppose to use it in screens which have a clear 'primary' purpose. In pretty much any email application, that'd be a compose email button from an inbox (or similar) view. So I guess in that particular case, it's not so much GMail taking from Inbox, so much as both of them are showcase material design apps, and Inbox just got released 'first'.
It's not clear that Inbox is the right way to look at email for the majority of users, and I'm saying that as a user/tester. I've had to drop back to the regular GMail app do do things a couple of times and Inbox has crashed twice on me.
Note that the GMail client did take on some of the look and feel such as the (+) widget on the bottom right.