Hi guys, once I had the same issue of my 1st generation GT Grade Carbon. The Answer of this problem is quite simple, it's a loose headset.

I have experienced it a lot of times. This "severe vibration" or "resonation", could vibrate from the fork all the way up to the handle bar, then up to my mouth, my teeth, then top of my skull... If you have experience it, you'll know what I'm saying.
It's really frightening, since most of the time this happens at steep downhill, and you could easily lose control when this happens, so once I called it "death vibration"...To trigger this, you have to go downhill, then press the front brake abruptly -- not in a gradual way, it's like to give the front fork a 'kick', and then you'll have it...

The cause of it actually is quite simple, when your headset is loose, the steer tube of the fork will have some wiggle room at the bottom area of the head tube, it's like a chopstick (or coffee stir spoon) in a long narrow glass bottle upside down, when you brake, the chopstick (the fork) are pushed backward, it will clash to the wall of the glass bottle, then bounce back and repeat. Your fork is like a rubber stick in a very slim bottle, you put the bottle upside down, put the stick in, let the bottle rest on top of it, give the stick a kick (sudden move or sudden stop), and you'll have the vibration.

Every bike with a loose headset could have this problem, but some are more prone to this, and some are not.
GT Grade in special, is more prone to this problem because the fork is built with compliance in mind, so it's quite bouncing, actually you could feel it when you spin the front wheel, and quickly press the front brake, you could notice the fork vibrates like a pitch fork. Then it's the disc brake, as the physics goes, the disc caliper is installed at the far end of the fork, opposite to the rim brake, that gives the move more leverage than a rim brake, which makes it more likely to stimulate this vibration.

In addition to all these, if the vibration won't trigger a resonation, it will not be too much of a problem, it will go quickly. But on GT Grade also my wild guess is the inherent vibration frequency of the frame and fork (high school physics, same as why your car vibrates severely for a moment after the engine start/stop) is somewhat near the frequency of this vibration. So when this vibration is triggered, it resonates and become a huge one.
All these add up, you finally have this "death vibration".

To solve it, it's quite simple, re-do your headset, loose the bolt of your stem, tighten the stem cap bolt, so you can still turn, but there's no wiggle room of the front wheel. Check Youtube there are a lot of videos about it, keyword: loose headset.

If you still cannot solve this, then maybe you have to do it further, un-do the compression plug in the steerer tube, the one you see when you remove the stem cap and its long bolt, pull it out, then re-install it to the specified torque using a torque wrench. I found mine not set tight enough by the factory.
If you still cannot solve this, maybe you should check if you have a manufacturing fault in the headset, the down side of the head tube maybe just too wide.... So even if you have it tightened enough, there're still enough wiggle room to cause this vibration.

Since this thing is relate to ride safety, so I replied these long words... In case anyone found this topic through search. And finally apology for my language... English is not my native one... I believe anyone have this problem should still understand what I said...
Hope this helps!