"Take the key and free a Merchant, who will tell you to meet him in Claywich, then loot the camp."

This merchant is bugged as hell: he might not show up in Claywich at all, or show, but you cannot speak to him. As a result, either you need to look for a save before completing this event and try again. Or, in my case, just walk with him to Claywich. And he is not marked on map. Stand near the mostwestern house.

"When you have that quest, play Gwent with the Trader"

For those who asked: guide suggested, that you play boatman much later.