Before doing anything else please check Gateway's site and make sure you can get chipset, networking, etc., drivers for XP for your computer. You may as well download them to a CD or DVD or USB flash drive now, 'cause you'll need them. Without those drivers you can probably install XP but you won't like trying to use it.
Part of your hard drive is probably a Recovery Partition. If so, make sure you have created a set of Recovery CDs/DVDs in case you ever want/need them. Also you probably don't want to destroy the partition itself, as it is not all that large and sometimes you change your mind about what you want to do.
Then buy your XP from a reputable dealer. Understand that if you buy a retail version you get some tech support from Microsoft and can transfer it to another computer. You can save money by buying an OEM version, but that makes you your own tech support and it's good for only the computer upon first installed.
Make sure your BIOS (Setup) boot order has the optical drive before the hard drive. Boot to the new XP CD. Format the partition that now contains Vista and install XP. Install the chipset, networking and other needed drivers; update Windows; set it up to your liking.