Any computer less than three years old that uses something besides an Intel onboard graphics chip can game to some degree. My mother-in-law's Compaq has a Nvidia 6100 onbord, and it does fairly well.
Our definition of a "gaming PC" is very different than what most people see as a "gaming PC." To us, it is a mid to high end rig with good graphics capabilities through recent GFX cards. For the public, it is any computer that can run games, or any computer that has a graphics card at all (no matter how old or weak).