Gale is one of the potential companions that can join your adventure in Baldurs Gate 3. He can be recruited just north of the Nautiloid Crash Site where you start your playthrough by helping him after getting himself stuck.
To unlock the achievement you'll need to adventure with Gale until you eventually learn he has an unstable Orb of Netherse magic inside him and needs to consume magical artifacts to avoid disaster, eventually a more permanent solution will be needed and at this point we'll need to travel to the Mountain Pass.
Traveling west out of the Nautiloid Crash Region map you'll find yourself on a path, traveling down it will see you encounter some strong undead enemies and should you be able to defeat them you can turn left on the path and continue on to find a familiar looking wizard named Elminster.
After meeting Elminster he can be sent to your camp where he will have an important message to give Gale, after the two of them converse Gale will be open to the proposal from his wizard friend and after a short scene Gale will have a new super special ability and your achievement will unlock!