Gaps when spread betting http://www.financial-spread-betting.com/Stop-loss-leverage.html refer to instances where today's market opening price for an instrument is different to the closing price from the day before for that instrument. When a normal stop loss order is hit, the broker will do their best to execute your order at the stop price. But usually you will get a price a tick or two from your stop loss, depending on how a liquid market you are in. This is called slippage. Both Gapping and Slippage are important to recognise as you can easily blow your account if you are heavily overgeared.
