Well, of the several kinds of coolant on the shelves at Rite-Aid, none of them said "G12," nor did they say anything like "E9" or "H17" that would be identifiably the same category of information. I guess that's what I get for trying to buy automotive supplies at a drug store.
I guess I'll give the dealer a ring and arrange to bring the car down, since the manual also warned that you shouldn't just add coolant, because the coolant warning could be a sign that there's a leak somewhere. It says you should bring the car to the dealer. So, I might as well just let them deal with it.