First, are you certain the problem is in the physical button? Ie. Does the button physically feel loose and engage intermittently or have you removed the panel that holds the buton on the computer and manually pressed to on off switch below it to see that the computer powers on and off like it should?

If you are sure it's the button and not a deeper problem in the system itself, then try emailing Gateway Tech support. http://support.gateway.com/support/contact/contact_tech1.shtml. If this is considered a 'user replaceable' part then they will most likely be able to send a replacement panel. If the system is out of warranty then there might be a charge, though sometimes they will send items like this at no charge.