GMail has no mail import option through the web client. One thing you could do is set up an email client to download your Entourage mail via POP, set it so a copy of all messages are stored on the server, and then copy your messages to your GMail account this way. You could do this using Entourage.
Don't know why you'd want to move from a standalone client to web based email though, this is a downgrade in my opinion.