In the Forge Lands, he's in 2 places I know of. One is right by where Gorewood is, like the above user said. There is another inside of a building over in the Fjord. You know the lake there? Well, you go past a building when you got to the first construct you revive, he's in there too.
Unlocking the quest
The quest can be unlocked in two basic ways. The first one is to defeat Thane during [The Maker Warrior] side quest. After winning the duel, you can receive a commission to kill Gorewood along with three other monsters. The second way is to find Gorewood without the involvement of Thane, at which the quest will automatically appear in your quest log. You won't have to take the quest from Thane first, but simply kill Gorewood and go to the Ancient to receive your reward.
Find and kill Gorewood
Gorewood isn't an easy monster to track down, but it doesn't mean you should wait with finding him until later in the game. Gorewood is located in the [Weeping Crag] in Forge Lands - the instructions on how to get to him can be found in the section devoted to that location. Just to remind you, Gorewood lives on a small forest glade which can be accessed by an underwater tunnel (after defeating a Stalker).
Return to Thane
Return to [Tri-Stone], initiate a conversation with Thane and tell him you had gotten rid of Gorewood. The primary rewards for this side quest are 603 XP and 3000 gold, although killing Gorewood also gives you a new legendary weapon, Gorewood Maul.