We talked a couple of years ago about the Google J2ME (Java for Mobile devices such as mobile phones) application that you can download and install on your phone. With this application, you can run GMail on you mobile phone, and access your e-mails with your data plan (GPRS, EDGE or 3G).
To install the J2ME application, visit (with your mobile phone) the URL
If GMail recognises that you are using a compatible mobile phone, it will direct you to download and install the application to your phone. The current version of the application is about 260KB.
If you want to save some of your data bandwidth, you can change the User-Agent string of your Firefox (use the User Agent Switcher Firefox Extension) to one of a mobile phone, then visit with your browser. In this case, you can get the application from googlemail-nokia.jar (version 2.0.6/L2). If you can afford it however, it is better to install from gmail.com/app, because this would set a list of reasonable defaults.