I'd agree that it's generational, I'm 28 and I've never seen the term ‘gaming’ used to refer to gambling until I entered this thread. No one in my generation makes that association! On the other hand, I've also never heard of the term used to refer to game development either, in that context I would assume it was used in error.
My guess would be that the term ‘gaming’ in gambling arose from the term ‘games of chance’, and that the two usages arose independently from their connections to the word ‘game’.