Dear Mik, Dear list,
Was it Walley who initiated the word “gamble” to use in place of the classical “random variable”? […], but I have not seen it anywhere prior to Walley’s 1991 book.
It dates from prior to his book. Already together with Fine he uses the term in their '79 paper “Varieties of modal (classificatory) and comparative probability” <https://link.springer.com/article/10.1007%2FBF00869449>.
Furthermore, Fine uses the term in his '73 book “Theories of probability: an examination of foundations” <https://books.google.com/books?id=U9lEAAAAIAAJ&q=gamble>. I don't have access to the book's full text here, so going back further in this line is for someone else. So, no, Walley did not initiate its use.
Of course others have used the word ‘gambling’ in the context before (e.g., Smith in '61 <https://www.jstor.org/stable/2983842>), but I guess you're focussing on the term ‘gamble’ as formalized by Walley, then for now we have Fine '73.
Best,
Erik