Where good-til-date measures in days, GTT measures in minutes. It is a way to encode "I want this fill in the next hour or not at all", which is often the honest version of a trade idea.
It is also a risk tool. An order left working into a scheduled event can fill on a spike you never wanted to be part of.
Example: it is 9:45 am and you want a pullback entry, but CPI lands at 10:00. A GTT buy limit expiring at 09:58 either gets you filled on the pullback or takes you flat-footed and safe into the number. See news-trading-rule if you trade a funded account where this is mandatory rather than optional.
Related: good-til-date, order-expiry