If I was long a stock and the stock was set to gap down, I would probably adjust my stop limit a bit wider in premarket, and wait for the gap to fill or partially fill before I exited. It really depends on where I am at in terms of profit or loss. If it was a stock that I had booked some nice gains on, until that dreaded gap down I may just let my stop get hit and take what profits I have and count my blessings.
I use Average True Range to set my stop areas. Usually 1.5 - 2 X the ATR on the daily, to have some wiggle room for swing trades.
Most gaps tend to fill at some point, so depending on the stock's particular behavior (and what news caused the gap down) I may ride it out and wait for it to fill before I exit a trade. It really depends on the stock. some good quality ones have very wide stop loss areas, around 10%, so I can ride the waves and collect the dividends and only get stopped out on a major breakdown.
The gap ups and downs on order entry/setups are what drive me nuts. Looking for ascending/descending triangles and double tops/bottoms and setting orders up to fill on breakouts/breadowns and then having the damn stock gap past my limit order is annoying as hell, but there's not much you can do except wait for the move to retest the previous resistance/support areas and then enter the trade.