Sadly I still encounter gimbal lock.
ASpaceGamePlayerController* PlayerController = (ASpaceGamePlayerController*) GetWorld()->GetFirstPlayerController();
FTransform Transform;
FQuat Rotation;
FVector MouseLocation;
FVector MouseDirection;
PlayerController->DeprojectMousePositionToWorld(MouseLocation, MouseDirection);
Transform = this->GetTransform();
Rotation = MouseDirection.Rotation().Quaternion();
Transform.SetRotation(FQuat::Slerp(Transform.GetRotation(), Rotation, 0.1F));
this->SetActorTransform(Transform);
Thanks for the suggestion, though.