In over 40 years, Gentleman has produced over 100 different Great Britain stamps . His earliest Royal Mail stamps were the 3 stamps for 1962 National Productivity Year.
Gentleman certainly has his style of designing stamps, as revealed from this interview by Gordon Milne, and in this article too. Perhaps his most adventurous idea ever suggested was to abandon The Queen’s full-face portrait (as this often caused design restrictions seen in stamps of the 1950s & 60s) and in favour of a profile in silhouette. The idea did survive and was first implemented in the 1966 Landscapes stamps.