You will only get this quest if you spare Gaetan while doing the "Where the Cat and Wolf Play..." quest.
Gaetan probably hadn't expected that Geralt would spare his life. Out of gratitude, he told Geralt where he'd hidden a stash of goods and said he could take from there whatever he wanted. Then the two witchers parted ways - and never met again.
Go to the place marked on the map. There you will find three bandits (25) who are trying to get to the Cat Witcher's Stash. Kill them, go inside and take the contents of the chest.
One of the items will be the witcher's steel sword - Teigr.
It turned out local bandits had found Gaetan's stash first. Yet before they could seize it and liquidate its assets, Geralt liquidated them.
