There is no word limit, however if you write a 300 word essay for example that would get marked terribly. I would recommend writing anything between 800 - 1200 words. If you can write 1200 - 1400 words without rambling and running out of time, than by all means write that much! Personally, I know that I can put across my argument well within around 1000 words, anything extra is seemingly unnecessary.
What? The English GAT is used to determined your study score?!?!?!??!!
It determines the top end students I believe (48 - 50), also if anything happens and you cannot sit your exam they will determine your score via your GAT score. It is taken into account when issuing all study scores though.