If you've got a googlemail.com address, you'll get the option to change it to the original gmail.com format...
If you go onto a PC browser, and log into your email, click
Settings - "Accounts and Import" - next to the email address - it'll give you a link to "Switch to @gmail.com?"
It won't affect emails you receive, you will get all emails sent to
you@googlemail.com but also you'll be able to send and receive to
you@gmail.com
It's just a nicer email address...
You'll have to change the address in the settings on your mobile though... the password / servers etc. won't change, just your email address