There is a set of instructions on Google's web site detailing how to set up a Windows Mobile email account to connect to Gmail over IMAP.
This change has no effect on Google's POP3 service.
What Is IMAP?
IMAP gives users more control over messages on an email server than the less-advanced POP3 service does.
For example, with IMAP, users can view the contents of multiple folders and make changes that immediately affect messages on the server.
The simplest and most common use though, is to delete unwanted messages on a remote device and have them removed off the server.
There's more information on IMAP on Google's web site.