Instead of getting angry here why don't you contact GAW support and try to resolve this matter in civilised way? If you contact them I am sure they tell you what happened and will resolve your problem.
This is always the best first thing to try. I would speculate that GAW likely overpaid for some number of blocks for some reason (maybe orphaned blocks) and the negative amount is due to this, or maybe they previously allowed you to withdraw more then they should have.
There are a number of logical explanations as to what happened, the correct one of which can be best given by their customer service/support team