What does Gatehouse Goods reward?

Gatehouse Goods gives Dalaran Sharp outright, 800 experience.

Where does Gatehouse Goods start?

Elaine Trias in Stormwind City offers it.

The quest log lists it at level 15.

Yes. Its quest id is not in Classic Era, so it is part of what Forever adds.

World of Warcraft: Forever · quest · level 15 · new since Classic Era

A level 15 quest in World of Warcraft: Forever, offered by Elaine Trias in Stormwind City. Recorded by 2 players running the addon.

Item	Given	Item level	Vendor sell
Dalaran Sharp	outright	15	6c

What does Gatehouse Goods reward?: Gatehouse Goods gives Dalaran Sharp outright, 800 experience.
Where does Gatehouse Goods start?: Elaine Trias in Stormwind City offers it.
What level is Gatehouse Goods?: The quest log lists it at level 15.
Is Gatehouse Goods new in WoW Forever?: Yes. Its quest id is not in Classic Era, so it is part of what Forever adds.

What sells at this level, and what to keep from these rewards, is in the level 10 to 20 gold guide.

Source: what players running the addon saw in the quest windows; the quest text itself stays in the game. Every quest seen · Stormwind City