Obviously gnome druids and other extreme classes shouldn't be playable. But we've seen and interacted with some odd-people-out that are weird combinations that are obviously canonically possible, like the Night Elf Paladin in Legion. That quest should simply unlock the combination for your account. They'll still be rare, a newbie could still see one and be all "Wow how are you a Night Elf Paladin?!" but they're possible, so they should be unlockable.