It is called GLOC, but it stands for G-Induced Loss of Consciousness. Most of what it seemes everyone is referring to is a "gray-out", not quite a blackout due to GLOC, but certainly the starting phase. It is basically just the result of blood draining from your head/brain, and thus your vision begins to fade out. It's not really dangerous, unless it's sustained for a long time, in which case your brain is getting no blood or oxygen, but that would be more like a couple minutes I would assume (not 100% sure about the time). In general, 3-4 sustained G's (several seconds) or spikes over 5-6 G's can cause people susceptable to GLOC to blackout, however, shortly after the forces stop, blood returns to the brain and you come to again. This is why few coasters top 5 G's anymore.
I myself have experienced this a few times, but only very rarely, and if you can predict it ahead of time, you can prevent it using the same method aviators who fly jets use (essentially you tense all the muscles in your body, effectively forcing the blood to stay in your head...it's a bit more complex than that, but that's the basic idea).