He was a paper-hangings manufacturer, of William Gentles & Co, paper manufacturers, importers of picture frame and decorative mouldings etc., brush and glass merchants, dealers in paintings, oleographs and chromos, at 38 Sauchiehall Street, Glasgow.
He lived at various times at different addresses including 18 Newton Terrace, 2 Belmont Street and 130 Elderslie Street, Glasgow.
Gentles was among the original lay-members of Glasgow Art Club in 1886. He was among those Glasgow painters and architects who in 1891 appended their names to a list requesting that the Corporation of Glasgow buy JW's Arrangement in Grey and Black, No. 2: Portrait of Thomas Carlyle (YMSM 137) (#12326).
Records of Glasgow Art Club; Post Office Glasgow Directory for 1890-91 and 1891-92.