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.

I think mainly your mail won't go into the sent folder and be searchable. You can work around that by CCing your domain.

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.