In case of a gap-up opening, if you are taking a long position, the stop loss for the trade could be placed a few ticks below the start of the gap.
For instance, if the high of yesterday's candle is 110 and today's opening is 112, we have a gap-up. If you are looking to take a long position, the stop loss for that trade could be placed a few ticks below the start of the gap, which is below yesterday's high of 110. The same logic applies to a down-gap as well.
Alternatively, you could also identify historical supports and resistances and use them for the placement of a stop loss.