Fade to Silence

Gani is an event based follower. When you are first exploring the zones the game will add an exclamation point to the map, and you'll be prompted that you discovered an event. You will know this is Gani's event if you see a hut made of wood near an altar.

Interact with the altar and Gani will confront you. She will want to do a ritual. Do the ritual with her and then hide together from a blizzard event. Afterwards you can invite her to your group and the achievement will pop.

The followers who join you via events are somewhat randomized. Gani's event may take place in the Prairie, but for your play through another follower event could be there instead.