the asx banned NAKED short selling, that is, selling without borrowed stock (and still is banned, covered short selling is legal).
The retail brokers will usually have inventory that u can borrow; Anyways be aware that the guarantee stop loss is usually a set % away from the last price. For the banks around 2%, for the more volatile stuff, 3%+
Considering gaps > 3% are quite rare, it may not be such a good deal. Depending on the provider, they also charge u around 0.5% of total trade value to use a guarantee stop. So make sure you do your homework before signing up for the cfd provider's guarantee stop loss (or a limited risk acct as CFD providers call it)