That is basically just saying "the gmail app can be a smtp/imap client for a 3rd party mail provider". It's not really using gmail as when the gmail app is operating in this mode there's a tone of gmail features you don't get.
What other features will you not get? The mail composer in Gmail is pretty sucky anyway. For example when you are replying on a long thread it is very hard to see the message you are replying to.