Hi Fred
I also had the trouble with going into minus balance on GTT...
What happened with me was...
I was sitting there for hours waiting for a certain user, to whom used to blast the buys/sells and I could actually make a few bob off him, I was buying lots and selling them to the same user for a cent dearer, lol...don't ask me why he was doing this, but he did win the Top 10 award two weeks on the run.
Anyway, as I started trading, I noticed my balance going into the minus and thought thats wrong.
For starters, there is NO possible way to go into minus credit, cos as you know, you CAN'T post a buy unless you have sufficent funds to cover the buys.
The only way around this was to take some of my current sales off and sell them for the highest on offer, something I was not happy doing, but was the only way around it.
I worked out that a couple of dollars had actually gone missing out of my account during this mess-up, so I mailed them, they looked into it and gave me double of what went missing to compensate.
Although this was good of them, I was still pretty well pis**d off, as this episode stopped me making a fair few bob trading

So my advice is to mail them, they WILL get it sorted, but it may take a few days. If you have any current sale bids up, take them off and sell for the best prices on offer (currently a pathetic 6 cents a lot), sell just enough to get your account back in the positive...
Hope this helped
Brian