You will be able get your gmail through the native iPhone program for it. The best thing to me about the Google App from the App store is the voice recognition. As long as you speak clearly, you can give the name of a restaraunt, a newspaper, etc, and it will post your voice query as if you typed it. It is very handy when on the road.
From there, if you are for example, looking for directions, you can hit the get directions button and it will launch google maps and you can use your current location to get turn by turn directions right on your phone.
The apps within the Google App tool simply launches a new safari window as opposed to opening up within the Google app. It's not what I had in mind, and I am sure Google is working on that.
Yahoo's app is free and has similar features, except that it opens up within the Yahoo App as well as giving you the option of opening it up on iPhone Safari. If I didn't have all my stuff on Google, the Yahoo app would work better as an app within the iPhone.