Whoever had Killing Gloves of Boxing in the Heavy Update Sweepstakes, you're the winner!
The KGB (fnarr) are the first of three unlockable weapons for the Heavy, and Valve describe them like this:
"The unlockable for the Heaviest pugilists out there is the K.G.B. Any time a Heavy kills an enemy with the K.G.B he receives five seconds of guaranteed critical hits. This crit-boost can be used to punch and kill further enemies, extending the boost time. Alternatively, the Heavy can switch to another weapon and use the boost to take out an enemy at range. The Heavy's slow movement speed, and the K.G.B's slower swing speed, make it a risky prospect."
Also revealed are the names of the Heavy's achievements, including tantalising offerings like, "Red Oktoberfest", "Redistribution of Health", and "Photostroika". (And yes, there is a "Soviet Block", obviously). The rest can be seen here.
So what's next?