I want to try games where the outcome is determined not by a random number generator, but by stock prices or other external financial indicators. Does this even qualify as gambling? And how is the fairness of such games monitored if the data source is outside the casino? Are there any pitfalls, such as streaming delays?

Games based on external rates (financial spreads) are considered betting, and their integrity is monitored by independent data providers. Stream delays are a risk, as odds may lag. I wouldn't recommend them, as it's difficult to verify their integrity. For example, the https://abigcandyplay.net/  website doesn't offer such games. If you prefer, look for specialized exchanges.