There's lots of references to all three publications in Barrie Rickards excellent boigraphy"Richard Walker" yet no precise dates for their closure.
In a personal memoir at the end, his 2nd wife Pat Marston refers to Grafham in 1968, and in the next paragraph writes about selling Fishing Gazette, which she edited, to a cousin, and then the company failed.These two paragraphs may not be in strict chronological order.
I was a keen reader of fishing magazines from the early 1960s yet at that time I never sawan Fishing Gazette on sale, or read one.
I'd guess that the FG ceased publication in the mid 1960s.
Perhaps Ron Clay will know more.