Gareth is first found fighting some Magisters on the second floor of a ruin at x467 y257. If you save him he will return to the Seeker's Hideout .
This will enable you to Remove your collars at the NPC's Leya & Duggan (x441 y17).
Gareth wanders around in the hideout and acts as a Geomancer & Warfare Dealer.
Gareth will move and make camp near the shriekers if you tell him you have a wand or if you have dealt with the shriekers already. See The Purged Dragon for more info.
He will mark his rallying point on your map. If you speak to him there, he wants you to deal with the Magisters ahead while he and the others dodge the fight and steal a boat to the Lady Vengeance.