Tap the Menu key.

Scroll to Installations and tap the Navigation key.

Scroll to Gmail and tap the Navigation key.

If it's the first time you use the application, you need to log on.

You can now use the application.
Please note: Gmail isn't part of the original application package and therefore only a general description is given here.
You have a number of options, such as:
Send emails
Retrieve and read emails
Reply to emails
View list of contacts
Select application settings

Tap and hold the Menu key to return to standby mode.