Ross, Herefordshire, is only about 25 mls from Llanwenarth. A John Gunter was baptised there in 1819, son of John Gunter & Ann Walling. They were married in Ross, 1814.
Is it possible, your John was born in Llanwenarth but baptised in Ross, where his parents married.
I know its a long shot but worth a look?
2 other children baptised there were, James 1816 & Amelia, 1821 to same parents.