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At seven years old, I'm going to venture that your power supply has died. That can be checked fairly easily.
There is another (much less expensive) possibility and that is that your CMOS battery (small watch type battery on the motherboard) needs replacement. I am not sure about Gateway but I do know there are some Dell models that will not boot if that battery is bad.
I will suggest spending the $3-$6 to replace the CMOS battery and seeing where that gets you. Let us know what you get then