Go to the graveyard outside Gadgetzan, and use the Videre Elixir. Follow the rest of Donova's instructions to find Gaeriyan.

Chain

It's a Secret to Everybody → It's a Secret to Everybody → It's a Secret to Everybody → The Videre Elixir → Meet at the Grave → A Grave Situation → Linken's Sword → A Gnome's Assistance → Linken's Memory → Silver Heart → Aquementas → Linken's Adventure → It's Dangerous to Go Alone