|Kuwese's link is for laptops and the other downloads require the gateway be working. I sense that might not be the case.|
Really all you need to do is open the thing up and have a look. The motherboard and any cards that came with it originally will have gateway part numbers on them on a sticker.
It used to be you could go to gateway's support site and punch in the serial number on the back of the case to get all that information. Well I think you can still do it but it no longer recognizes some of the older models. You can try this and see what you come up with:
Otherwise you'll need to give us more information than just 'gateway 450'.
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