GST means "Goods and Services Tax" and it is a tax levied in Australia at 10%.
If the ad says Inclusive of GST then there nothing else to pay tax wise. It is tax paid or no tax needs to be levied. However if you are from another Country outside of Australia then you need to know your own tax laws and if custom duties will be charged on your side. This you need to find out for yourself.
A couple of bullion companies here might put in their ads + GST, that will mean final price reached at auction + 10% GST. They will say if there is GST payable or not. If they don't say whether there is GST payable or not then don't be afraid to ask as they are very easy to get along with.
99% of sales here are peer to peer and there is no GST. So when you trade with other members here there is no GST payable, we are mostly all hobbyists and just buy/swap/sell each others metals. I hope that makes it a bit clearer, cheers.