Ignore what everyone else said. Many of them don't have a Gateway, and don't know what they're talking about.
The bare minimum you need to do to upgrade is replace the launcher.dat on the root of your 3DS's SD card with the latest one (3.7.2) on Gateway's website.. If you're unsure of the steps, use the quick-start guide included in the download.
I recommend going a step further and installing A9LH. To do so, first follow
Plailect's guide (moving emunand is optional), then follow
this guide to get the Gateway card working again.
You're a little late to the party. I've already had a discussion about why there's no point in a Gateway owner abandoning Gateway.
(and many more quotes about why running 3DS ROMs is useful)