Slippage on gold is the difference between the price a trader requests and the price at which the XAUUSD order is filled. It is driven by liquidity and latency: at a data release resting orders are withdrawn so a market order walks further up the book, and in the milliseconds between click and fill a metal capable of moving five dollars in seconds has already repriced.
Stop orders slip for the same reason, because a triggered stop becomes a market order, meaning a three dollar planned stop can fill four dollars away during CPI, non-farm payrolls or FOMC. The practical defences are limit entries, avoiding market orders inside the release minute, sizing that survives a fifty percent stop overrun, and logging requested against filled price so slippage enters the expectancy calculation as a measured cost.