You can't, well not natively at least, since filemaker will use your system mail client, not a browser based client. You might be able to fool windows into thinking your gmail account is the default. Try these articles:
You CAN use the SMTP settings from your gmail account with the SMTP send mail script options in Filemaker.
If you need more robust mail functionalities, you should consider a plugin, such as 360works email: http://www.360works.com/email-plugin/