That "stuff" is polymerized paint, with the pigment. From your description, it is not recoverable. Some paints just settle hard, but you can redisperse them with thorough stirring, NOT shaking. But if the stuff is ropey, gooey, and fairly coherent, your best bet is to discard it.
It did it because that's what paint does—it just did it too soon. It is possible that the paint is very old. Who knows how long it sat in a warehouse, on the shelf in the store, etc., or what conditions it experienced in transport or storage before you got it? Cooking in a semi-trailer for a few days in the heat can do it. It sounds like there was a loss of solvent. The bottle should have been essentially full. Solvent in a paint does two things, basically. It reduces viscosity, and it slows or prevents polymerization of the binder until it evaporates.
If you can, take it back for exchange—the cost of model paint is cosmological, in the Carl Sagan-ist sense.
Some brands are more prone to this than others. One of the reasons I'm so fond of Tamiya and Polly Scale is that they seem to have slightly fantastic long term stability (and prices to match). I've heard Gunze is similarly resistant to the ravages of time and elements, but have little experience with it, as it didn't seem to fit my painting methods (strictly a matter of taste, here).