An ordinary stop-loss promises an attempt, not a price. A guaranteed stop shifts the gap risk to the broker, which is why it is the only order type that genuinely caps the loss on a leveraged position through an event like a weekend-gap or a swiss-franc-unpeg-style shock.
It costs money. Firms charge either a premium at the time of placing, refunded if the stop is not hit, or a wider spread on the position, and they impose a minimum distance from the current price. Not every instrument is eligible, and availability is often withdrawn ahead of known events.
It is worth pricing against the alternative: for a trade held through an election or a central bank decision, the premium is frequently smaller than the extra margin a trader would otherwise post to survive the same move.
Example: a guaranteed stop costing 0.3% of a $20,000 position is $60. A gap that fills an ordinary stop 90 pips beyond its level on that size would have cost several hundred dollars more.
Related: stop-loss, weekend-gap, slippage, stop-level-distance