According to the Pipephil.eu page provided at the end of the text,Gardhill pipes are manufactured by the J. Vincent Fils firm at their facility at 11 rue Carnot in Saint Claude, France. The history of this firm dates back to 1898 when Henri Vincent-Genod first had his workship at his house. Slowly, the company continued to expand first in 1909 when Léon Vincent-Genod, Henri's son expanded their workshop , as well as in 1923 when Léon's son Jules took over the Verguet-Buffet pipe factory. In 1927, the company became formally named the J. Vincent Fils firm, which was located at the 11 rue Carnot location. The Gardhill brand took up most of the firm's production capacity around the early 1990's. In 1993, the firm closed down.
For the source of all information provided, see the J. Vincent Fils firm page on the Pipephil Website