Not sure about Gateway, but most OEM's simply provide a "factory" restore CD for their systems, which are only valid for that specific OEM model. However, if the CD you have has a holographic image with "Microsoft Windows XP" on it -- then you may be able to use that with Boot Camp. Nevertheless, it all depends on if the XP install CD you have includes a Product Key.
By the way, it is Microsoft's intention to discontinue the retail sale of Windows XP by the end of June 08. So, if you want a legal copy of XP, it might be best to buy one now.