It is an unusual site, the garden of a former farmhouse which stood beside the Corwen to Bala branch of the railway where it crossed the Dee. The disused line can be seen continuing in the trees at the middle right of the picture and one of the stanchions of the bridge forms an island just visible above the gate.
Hubbard  says of G. maxima "....of value on river banks for the prevention of erosion." It grows on the bank just behind the gate, with Phalaris arundinacea, Reed Canary-grass, which Hubbard says also helps prevent erosion. I wonder if they were both planted here for that reason?
The willow on the left was certainly planted during the present owners' tenure [c. 25 years], and has been identified by Desmond Meikle as Salix x fragilis L. with its partner [out of sight] as the Bedford Willow, Salix x fragilis L. var. russelliana (Sm.) Koch.
Chater, A O, 2010, Flora of Cardiganshire, Aberystwyth
Hubbard, C E, 1984, Grasses, 3rd Ed revised by Hubbard J C E, Penguin Books