Travel to Gatehouse from London

Get a train from Paddington main line to Newport (South Wales), which is 2 hours max. Change and get a train to Chepstow Station. 

You need to pre-book a taxi from the station.   (it is approx. 2.5  miles from the station). 

There is a pub within walking distance (10 mins) and a farm shop selling everything (meat, cheese, milk bread, chocolates etc) about 1.2 miles away.