Assume you are using the right way of switching it on -- physical switch on the case or Fn key combination ? So check if it's present in Windows Control Panel, System, Hardware, Device Manager or whether it's yellow flagged as faulty.
If so, try reloading the driver for it (on the CD which came with the computer or maybe a download from Gateway). If no joy, may have to replace (if it's a mini-pci card under a flap on the bottom of the case). Safest to buy a new mini-pci unit from Gateway -- if that's a silly price or the wifi is built into the motherboard, you may have to settle for a USB wireless adapter.