Put this in your ~/.fetchmailrc:

poll imap.gmail.com protocol IMAP user "(insert username here)@gmail.com" there with password "(insert password here)" nofetchall keep ssl

Then have xbiff use this X resource:

XBiff*checkCommand: fetchmail -d0 -csk

I also recommend adding this resource to prevent it from checking so frequently:

XBiff*update: 300

Now xbiff should check your Gmail inbox. I prefer to use it in FvwmButtons.