Google allow you to install a Java MIDLet (a small Java application for mobile platforms), which gives you access to your GMail account. Current version is
The advantage of the MIDLet over web is mainly speed and more mail-oriented interface. And compared to the Mobile Outlook it has the advantage of not destroying e-mail when you download it.
However I am unable to use it productively, since the MIDLet manager, installed on my T-Mobile Vario I has very tight security settings. So every time the application tries to access the Internet, it pops up a warning Yes/No dialog. And, if you think about it, almost any operation requires network access - checking for new e-mail, sending messages, moving them and so on.
If Google bothered to sign their MIDLet, then, probably, the MIDLet manager could be convinced not to be so paranoid, but for now I have to carry on using the web interface.