Well, looks like it’s good news/bad news. I decided to take it into the local Chevy Dealer as I found the local independent I was using pretty much charges the same as the dealer (plus, the dealer may give me a free loaner if the car can’t be finished today).
Good news is it’s not the head gasket. Bad news is it’s the lower intake manifold gasket. Also bad news is I recently had the engine flushed and refilled and changed the oil (Mobil 1), so they’re going to need to replace the oil and antifreeze, too. So $1300 - $1400 with the flush they recommend (as I had the green antifreeze in and they need to flush it out well).