GTT is a Limit Order, it won’t execute beyond the price you have specified.
A limit order allows you to buy or sell a stock at the price you have set or a better price.
If market opens gap down at 250, your order will not execute and remain pending. Sell Limit order will execute at 290 or above (better price), not below 290.
If market opens at 450, your Sell Limit order will be executed at best available price in the market. Above example, Sell Limit order will execute at 380 or above (better price), not below 380.