From the question its not clear if you added a mail account to the default Mail app. If yes, I don't think there is any such issue.
Also there are no official apps from Google. What you might have downloaded is an app offered by another developer who mostly might have exposed a Web page from the app.
The best way to use Gmail is from the Browser. You can pin the website on your start screen and this will start acting like your "App" (without the necessary notifications).
Answering your second question about editing SMS when forwarding - when you forward a message, you do get an text box where you should be able to select/add/modify/delete existing message content. Check out the steps here - http://www.windowsphone.com/en-us/how-to/wp7/people/send-a-text-message