I never have seen anyone who wouldn't agree that the gromril great helm gives an additional point of
AS.
The text states that the helm may be worn in addition to normal armour, so why is this even a question ?
Page 43 of the
BRB shows how to calculate AS. It states there that there may be other modifications to armour saves apart from light/heavy armour and shields, mentioning - but not limiting it to - riding a mount.