Your printer is attached to Outlook Express because it's the only default email application on your computer. Gmail is a web-based service, not an e-mail client and what you are asking for is impossible. When composing an e-mail in the Gmail interface you need to manually browse the files you want to attach. There is no known application to do that between a printer and a web based email service.
On the other side you can still use Outlook Express with a Gmail account and when you want to perform a printer-related action it will still use Outlook but when you want to send the email, you'll use a Gmail account. These instructions will provide sufficient information on how to attach a Gmail account to Outlook Express.
