Hello,
Judging by yesterday's events, many people do not know how to set a stop limit sell-order vs. just a stop loss order. In the latter, once your stop price is breached, your sell order will execute at the next available price. Given yesterday's flash crash on GDAX, many people's stop loss order triggered at very low prices once the price fell from above $300 to approximately $224. And the stop-loss orders continued cascading down causing the price to fall as low as 0.13 cents.
If you don't want to just HODL, and want to protect yourself against downside risk, you need to set a stop loss order.
In GDAX, select sell and click on Stop. Enter your stop price. Then click on advanced and select your limit price. In the example above, my stop price is $300, and my limit price is $290. If as yesterday, the price gaps below the limit $290, the order would not execute. This would have prevented many users from GDAX from basically giving away their ETH at unreasonably low prices.
This is pretty basic, but again, given yesterday's events, it seems that many people did not know about it. Happy trading and be careful out there!