If it was "almost the red mark" I'd be surprised if that was enough to warp the head and require a head gasket.
However you could have a blown head gasket causing the water leak in the first place but it sounds as if that was down to your radiator.
There are holes for oil and water and the cylinders in a head gasket
This is what they look like.
http://www.allproduct.../yingtai/product5.jpg
Water can get into the oil - this would give you the white sluge mentioned.
Oil could get int the cylinders - this would give you black smoke from the exhaust
Water could get in the cylinders - this would give you white smoke from the exhaust.
A mechanic should do a compression test of the engine to diagnose a suspected head gasket - you should really ask for this to be done before they pull apart your engine and give you a big bill.
I've yet to meet a mechanic who changed a head gasket and then told the customer actually it turned out it didn't need it