For PC:
The most useful gift for a PC gamer is probably a trip to the chiropractor or a proper chair to sit in for hours of gaming and computer work but for something a little more fun try these out: Diablo 3, Mass Effect Trilogy, StarCraft 2 Wings of Liberty.
For Any Gamer:
At a cool $300.00 the Astro A50 Headset comes with a hefty price tag but audio is something that can make the difference and this set is compatible with a PC, Xbox 360 or PS3
http://www.ebgames.ca/accessories/universal-astro-a50-wireless-headset/315402