I think you've captured a really interesting perspective on Watson and his relationship with gambling! It's true that while Holmes had his own set of vices—smoking, drugs, etc.—his more “temperate” habits, like alcohol, didn’t seem to dominate his life the way gambling might have for someone with his level of intellect and curiosity. Watson, on the other hand, does seem more like the type to engage in social activities like club life, and it's easy to imagine him getting drawn into the occasional wager or game.
It’s an intriguing thought about how Holmes might have operated in the world of gambling, though. His strategic mind would undoubtedly give him an edge, especially in a game like poker or cards. I can just picture him calculating odds and reading people in a way that would make him nearly unbeatable at the tables.
As for the gambling debts, I think you’re right—if Watson did end up losing money in a game, it would likely be because of a bit of risk-taking or overconfidence in his ability to control the stakes. And I could definitely see Holmes stepping in to protect Watson from himself, just as he often does in other situations.
The whole idea of Watson slowly stepping away from the clubs as his adventures with Holmes intensified is also really fitting. It speaks to the bond they share—adventures and detective work taking precedence over more typical social activities of the time.