How do I change the home screen's Email icon to go to GMail instead of the default Email app on the Samsung Galaxy S?
click "applications", click menu button, click "edit". You should now be able to drag the email icon away. Then just replace it with the gmail icon by dragging it to its place.

The other way is to configure the mail application to connect to gmail with imap or pop, but I wouldn't recommend it since gmail app is much better.

Hope this helps!